I'm looking in my blue book here, and I see it says the model 1911a1 had the "C" prefix until 1950, and then the "C" suffix started in 1950.
It says the "C" suffix went from 1950 to 1970. It lists the serial numbers as 240,228C to 336,169C.
This was on what they called the 1911A1 Colt Commercial in my blue book.
Hope this helps some. The book I'm using is by S.P. Fjestad.
Jimster