U.S. Mark II Fragmentation Hand Grenade, WWII and After

Mk.II Cut-Away View Mark II Grenade Construction and Function
The "mouse trap" fuze design, developed for the Mk.II in 1918, proved to be one of the most successful grenade fuze concepts. This type is widely used on American grenades as well as copied, in one form or another, by many other nations.
Its function is simple, cheap to make and reliable.

During the early years of WWII there were some problems with the fuze however. One was the flash from the primer sometimes blew past the delay fuse into the detonator directly, resulting in premature detonation. In tropical environments there was moisture entrapment under the foil seal. When used, there was a loud report, flash and smoke issued from the fuze vents, which attracted unwelcome attention in battle. So it is understandable that the Mk.II fuze went through a number of changes, accounting for the the different fuze markings found: M10, M10A1, A2 and A3, as well as the M6 series. The external physical appearance of the fuze remained constant as the changes were internal.

Fuzes are marked on the lever by metal stampings or black ink. Some are unmarked while others are metal stamped with one type and subsequently ink stamped with another identification.

The M10 series is the most common found.

Mark I Arming Sequence
Hinge Pin Variation
Fuze Mechanical Variation
The curved, fold-over hinge, safety lever design was used for about 25 years but it had an inherent problem. Under certain grip conditions, the striker can push the lever forward and off the hinge lip ("A"). Without the hinge engaged the lever can swing back, pivoting at the palm, releasing the striker with the grenade still held in the hand.
A quick modification to correct this was a striker pivot pin which extends beyond the bouchon wall. With a flat cut in it ("B") a lever stop is created. This prevented the lever from slipping forward. An interesting fuze feature to look for.

A better solution was the creation of the modern split-toggle hinge design developed at the end of the war and commonly found on modern fuzes.

Practice Mk.II Types
Practice Grenades - Mk.II and the M21
During WWII the practice Mk.II  grenade was made using a standard HE grenade body and fuze. The HE filler was removed, and the threaded plug was replaced with a wood or cork plug. A small black powder spotting charge was used to create smoke and report. The body was reused as long as it remained intact.

With the elimination of the filling plug on the HE body, towards the end of the war, a new practice grenade was needed. This led to the creation of the M21. (Discontinued sometime in the 1950's)

Shown is a 1953 dated example with the M205A1 practice fuze.
The "green" stuff is a yellow varnish applied to the base plug with the grenade inverted.  This was to seal the body from moisture. The grenade also contained a black powder spotting charge and were issued ready to use.

Fakes & Forgeries - The "RFX" Body
Do to its popularity for American collectors and increasing value, Mk.II grenades (and fuzes) are being faked with increasing frequency.
A common attempt at "creating" a WWII era Mk.II, uses the grenade body found on the M21 Practice. While original M21 grenades in good condition are becoming collectable, relic grade bodies can still be found. It is a simple matter to put a coat of OD paint on it and add an original (or repro) fuze to make a convincing display for the casual buyer.

As with many hand grenades, Mark II bodies with missing fuzes are common and original fuzes in nice shape tend to be hard to find. Not surprising a well made repro fuze has recently appeared in quantity.  These have been offered alone or combined on Mk.II bodies. Sometimes correctly described as reproduction, but often presented otherwise.  

RFX Body Comparison M21(Post WWII Fuze)  - Mk.II -  M21 (Re-painted)

In the photo is an original Mk.II body at center, with an original M21 on the left and a repainted M21 body on the right. Both M21 bodies have the letters "RFX" cast in them.

There are a few things to note here:

o The RFX has a distinct "knobby" frag segment profile.

o The hole in the base is large and unthreaded.

o "RFX" is stamped prominently on the side.

o "RFX" bodies were never used for HE grenades.

o M10/M6 style fuzes will fit the M21 but not the current "RFX" style bodies common on the surplus market. (See Below)

Usually attempts to create a WWII Mk.II facsimile are amaturish and easy to spot. Not many times is the public at large fooled, but it does create some confusion among novice collectors.  
Unfortunately there are more sophisticated efforts out there than can be pretty convincing.  As values rise, so will be the potential profit from a well done forgery.
Of course there are many sellers, offering grenades found in estate sales and such, who do not know much about grenades and are innocently just presenting what they believe is true. So often it is up to the collector to determine the facts.

Here are a few examples to consider:
A common misrepresented "Mk.II" type found currently.
These RXF bodies are not vintage M21 types, as the thread will only accept the modern M228 or M213 fuze shown. It seems these bodies are not even U.S. production.

A novelty item. These are of no value to any serious collection of American ordnance, as the M21 was the last official Mk.II training grenade.
Here appears to be a vintage M21 with yellow paint added and "aged".

A blue repaint would have been a better choice, as a restoration of the original M21. The fuze looks like the correct M205 practice, which are becoming hard to find.

As is, a pretty silly effort.

At left is a more sophisticated forgery.
(Yes offered as "original WW2")

The most obvious identification of the "RFX" body (other than the RFX stamp) is the large unthreaded hole in the base. The person who worked on this piece did a pretty good job of plugging that tell-tale feature. You can just make out a faint outline. (A quick look inside will no doubt  show the truth.)

The paint job was very convincingly done with an aged green over yellow finish.  If the effort included filling the RFX cast letters, (a curious oversight) this would have been a hard fake to spot from a photograph, although the RFX body profile was still evident.

There have been modified RFX bodies observed where the "knobby" segments had been ground down and flattened, an attempt to mask the characteristic profile..

A final look at a grenade modification that has nothing to do with forgeries, but worth mentioning.

These two grenades were advertised as "authentic" WWII Mk.IIs.
To a certain degree this is true as the bodies apparently started out as original but have since been altered.

These grenades had been modified into cigarette lighters. Note the oversized threaded plugs with the coin-slot cuts.
The bases were drilled and re-threaded to fit a removeable plug for lighter fluid.
The one on the right has modified fuze threads as well, with an insert to fit a lighter assembly. (Offered as body only.)
The one on the left still had original fuze threads, as a vintage M10 fuze was restored to the body.
Darryl Lynn has great detailed information comparing markings,body styles and more. You should check that out.
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