There are basically three types of USGI M1 carbine bolts:
Type 1 - Was the original production design. It had a flat top surface with the extractor area cut all the way through, leaving the bolt lug area weak. These early bolts were prone to fracturing which caused the right lug to break off. Only early Inlands and Winchesters used these.
Type 2 - Introduced in mid 1942 to replace the Type 1 bolts. this was a flat topped bolt with a reinforced web of metal under the extractor which solved the breakage problem experienced with the earlier bolt design. All USGI carbine prime contractors used this bolt at one point.
Type 3 - In late 1943, Inland experimented with the round bolt. The design proved successful and was adopted in March 1944. Erroneously called the "M2 Bolt", the new design eliminated a number of machining steps and created a better bearing surface with the receiver for a smoother operation.
"U.S. M1 Carbines - Wartime Production" by Craig Riesch is an excellent source for carbine information.