They did. They even issued field upgrade kits. A problem was that many converted M1 Carbines were still marked "M1", even though they were now actually M2s. Visually, the selector lever pretty much let you know what you had.

My Plainfield M1 Carbine has the stock "cut" for a selector lever, even though it was an M1 configuration, and the stock was not one of those later "pot bellied" versions. When my friend was putting together Class 3 weapons, he used my Carbine to test one of those conversion kits (affectionately called "overdrives"), and the selector lever fit into the stock perfectly. We tested my Carbine in semi and full auto, and it worked very well. The downside to me was that the muzzle rise on full auto was very noticeable.

Harvey