exactly what dan said , but ill add that neither the M1917 rifle nor the M1917 sling were 'first string' both were 2nd to the M1903 rifle and the M1907 sling were the US standard of issue , that i 'think' is why the two get paired so readily ,

ive no longer got a thompson sling but i do still have both an M1917 & a M1903 version of these , the latter is the shorter but also is missing the auxilery ring at the butstock end ,

im sorry my photos arent better but here is the M1917 rifle version , note the ring that the butstock [short strap] snaps to in its shortened position


and here is my M1903 version , note it snaps over itself