The C-100 - C-135 series was re-numbered to the 51 - 87 series starting in September 1975, due to retirements, the series only got finished to 81. The older buggies and the flattop transfer buggies were re-numbered into the 401 - 499 series starting in 1981 as were the now 51 - 87 class. It was said around the RR that the reason for the re-numbering was that the "C" and "W" prefixes did not work in the car tracking computer system we were using at that time.
If you would like a re-numbering roster, I can post it here in short order.
David Hutchinson