If your tank matches your crank case you have a C. Some of the early C’s do have square tanks. Rudder F, A, B and C’s flywheels will fit each other’s crank shaft.( also D’s) G’s, J’s, L’s and early Speedster’s flywheels I believe will fit each other ‘s crank shafts also. The carburetor appears to be later too. But remember they were always experimenting and it is not uncommon to come across motors that are stamped “Experimental “