The M1921, M1927 and M1928 Thompsons accepted either stick magazines or drum magazines. The stick mag was the 20 round "XX", and the drums were the 50 round "L", and the 100 round "C". In WWII, the army had 30 round magazines made, which were sometimes called the "XXX" magazines. (The designations, in case you missed it, are simply the capacity in Roman numerals.)
The 100 round drum was not used by the Army and was mostly used with the Model 1921, for which it was set up. Some 50 round drums are marked "wind to 9 or 11 clicks". 9 clicks is right for the 1928, 11 for the 1921, which cycles faster. 100 round drums today are very scarce and bring well over $1000 in good condition.
The Army adopted the M1928 as the M1928A1, but when the gun was simplified to reduce cost and improve production, both the Blish lock and the drum capability were dropped.
The M1927 was a semi-auto Thompson, which is why the modern semi-auto is called the M1927A1. The original M1927, though, fired from an open bolt, not a closed bolt as required by BATF for the modern gun. The change resulted in a need for a very heavy recoil spring since a closed bolt gun cannot take advantage of advanced primer ignition as an open bolt gun does.
HTH
Jim
