Developed during the late 1930's, the M3 Anti Tank Gun was essentially a copy of the German PAK 36, with minor modifications. Unfortunately, by 1942 that caliber was obsolete for use against modern armor in Europe.
Still, it served through the entire war and proved very useful in the Pacific theater.

A total of about 78 million rounds of ammunition were produced, of that, roughly 70% were Armor Piercing, 21% High Explosive, and 9% Canister.

For the collector, relative rarity of these rounds aligns with those numbers, with the M2 canister being extremely difficult to find.
Headstamp dates are most likely 1942 or 1943, which were the years of highest production.

M63 HE Shell: Almost completely filled with explosive with a base-detonating impact fuze.
M51 APC-T Shot: A tracer element was pressed into the base of the solid projectile.
APC ("Armor Piercing Capped") shot consists of three parts: solid penetrator, piercing cap, and topped by a ballistic cap. The reason for these parts is, at high velocity, the hard steel penetrator would shatter against faced armor. If tipped with a soft steel piercing cap the impact shock is reduced keeping the round intact. The hollow aluminum ballistic cap on top, provides a better aerodynamic profile.
Note that the German AP round (far right) is un-capped. Its slower muzzle velocity (2500 fps vs 2900 fps) did not require it.  (See 75mm Tank page)
M2 Canister: For close quarter use against massed infantry.
The "projectile" is filled with 122 3/8" (.38cal) steel balls and bursts open as soon as it leaves the muzzle. The effect is that of a large shotgun, with an effective range of 250 yards.

American tactics for anti tank warfare were about speed and maneuver. Rather than using fixed emplacements waiting for the battle to come to them, mobility was preferred to bring the fight to the enemy. Towards that end, the U.S. was fond of the S.P.A.T., "Self-Propelled Anti-Tank".

Shown is the 37mm M3 mounted on a Dodge three-quarter ton truck (circa 1943).