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  Mk.I Illumination Grenade 

Mk.I Illumination Grenades Mk.I Section View
Diagram Courtesy of the U.S.M.C.

This is the Mk.I Illumination grenade made and used by the U.S. during (and after) WWII. The two shown above are dated 1944.
The body is made of two sheet metal cups with the illuminating compound loaded in the lower half. When ignited, the grenade halves separate due to the combustion pressure. The grenade burns for 25 seconds producing 55,000 candle power. A typical use for the Mk.I was in conjunction with a trip wire in a defensive perimeter, as well as other battlefield illumination and signaling purposes.
The grenade shown below has been fired. The intense heat from the illumination compound has burned the paint and discolored the metal on the lower cup. Since the halves end up far from each other, fired grenades can be found with the upper section still in good condition .

Mk.I Fired

The fuze is a special quick match design with a seven second delay. (I'm not aware of any specific "Type" designation.)
Both fuze examples shown here are unmarked.

Mk.I Quick Match Fuze
Mk.I circa 1968 This is an example of a Vietnam Era Mk.I, dated 1968.
Designated "MK 1 MOD 2"

Is seems that the "Mod 2" uses a different fuze (M372) which eliminates the quick match design.
(Thanks Tony)