U.S. Mark I Fragmentation Hand Grenade, WWI

Mk.I Cut-Away View Mark I Grenade Construction and Function
Evidently the U.S. War Department was of strong opinion that young American males of 1918 would find pitching a baseball to be an almost natural act, hence, incorporating that "inherent" skill into the function of this grenade would be a clever thing to do.

To operate, the grenade was held in the right hand, the thumb crossing over the lever, the high ridge nested in the crevice of the thumb's first joint.
The pin is removed and the top cap shaken off the grenade.
With an over hand throw, the lever was intended to be "automatically" twisted, by the thumb, as the grenade rolled off the palm of the hand. At about a 20° angle the lever frees the cocked striker which snaps like a mousetrap, initiating the primer and lighting the 5 second fuze.

Soldiers were already familiar with the French F1 and British Mills, which only required a simple release of the safety lever, without any regard to throwing technique. Having to adjust to the curious mechanical features of the Mk.I was no doubt often forgotten in the heat of battle.
If operated as the other grenades, the Mk.I would be thrown in a safe condition. The enemy merely had to pick it up, twist the lever and throw it back. Since many German grenades required manual arming while still in the hand, the Mk.I design was probably more comfortable for their use than for Americans.
Mark I Arming Sequence
As mentioned, Mk.I production ceased almost as soon as grenades were issued.
The fuze was redesigned and the frag body modified, increasing the number of segments from 32 to 40.
The new grenade was designated the Mk.II.
However, existing Mk.I bodies were used as is until supplies were exhausted.
A transitional configuration of a Mk.I body with the Mk.II fuze evidently is the most common type issued up through the Armistice.

After the war, the body was again modified, taking on the more familiar profile found on later Mk.II grenades.

All variants have a 3/8 inch threaded fill plug in the base of the grenade.
Mk.I to Mk.II Transition
MkI - MkII Comparison

To protect against moisture, there was a foil disk, sealed across the top of the primer and the vents. Here you can see a unusual example of an intact disk, with a dimple where the striker has hit.

Also worth noting is the presence of spanner wrench lugs on the sides of the fuze body. That feature was originally absent on early fuzes as shown in the example above. It was added before the Mk.II development, so the Mk.I fuze can be found with or without that feature.

Mark I Practice Grenade
Mk.I Practice Grenade
For practice in handling and throwing, there was a special grenade produced for that purpose, the Mk.I Practice. It is cast iron and hollow with no functional parts. Interesting here, the lever profile of the Mk.I fuze is replicated, but the body is similar to the post war Mk.II design. Another feature of this example it that it is painted blue, which is a color adopted for practice types much later. It would be expected that it be painted black or unfinished.
Possibly this one was kept in use for many years after WWI. One of those curious anomalies out there.
Grenade Vest and Grenadiers ...
Grenade Vest In the closing months of 1918, there was anticipation by the Americans of a massive Allied offensive in the spring of 1919. Arms production proceeded at the maximum rate possible. Thoughts about the role of the hand grenade are reflected in this training manual excerpt:

"Grenades have now come to be universally regarded as indispensable both in attack and defense."
" The French consider that the grenade, has to a great extent, supplanted the rifle as an offensive weapon in trench warfare; thus we see grenadiers in the first wave of attack."
Grenade Training Manual - Army War College, Jan 1918

Training proceeded with the formation of elite squads of grenadiers, who were expected to lead the attack. Initial offensive contact was foreseen as developing into grenade duels between single squads or even single throwers. Complex assault tactics were developed.
With this in mind, ways for soldiers to easily carry large numbers of grenades were considered. Two, already used by the British, seemed most promising, the canvas bucket carrier and grenade "waistcoat" or vest.
Above is the 11 pocket U.S. vest which  hangs from the neck and held fast by four chest straps.
This one is dated May 1918. (Also shown is the M1905 Bayonet - Dated 1919, with the early Mk.II grenade.)