While the rifle grenade– pioneered on the Western Front in the Great War– was nothing new by the 1940s, the American M1 Grenade Adapter allowed GIs during WWII to toss standard fragmentation grenades (pineapples) rather than special rifle grenades downrange.
To point out how it worked, in great detail, is CSM(Ret) Rick Lamb in a great 20-minute piece over at Tactical Rifleman.
Enjoy!
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Awesome! I learned alot-details never known/seen before – thanks for posting.