hps1 is correct. The short M1 was a request by the WWII European command paratrooper generals, especially General Gavin. They considered the M1 Carbine as unreliable and underpowered. They wanted an M1 that could be jumped assembled, ready to go.
Two prototypes were built, one destroyed, and one on display at Springfield Arsenal. The rifles had excessive muzzle blast and recoil, and before a fix could be developed, the war ended and the project was canceled.
In the late 1950's the owner of a California surplus gun company had a stock of M1 rifles that weren't selling. After seeing the T26/M1 at Springfield, he decided a carbine M1 would help sell his overstock, and he converted a number of rifles. He coined the name "Tanker Garand" as a sales gimmick. The rumor started that the Tanker was requested by the Pacific command for issue to tank crews.
As a historical note: Klein's Hardware in Chicago had ads in Field & Stream in the 1960's, selling M1's in excellent condition for $99.99. P-38 Walthers were $39.95. Lugers were about $5.00 higher.