The MX-350 "S or R" versions are the only programmable MXs. It programs in the R-1800 suitcase programmer - the C & D versions of the codeplug are eraseable and re-programmable. The A & B versions are a one-time prom. "D" version firmware for the 1800 can program C or D version chips. "C" version firmware can only program C version chips. "B" version firmware can program A or B version chips. "A" version firmware will only program A version chips. Also the programming adapter changed from A & B to C & D. They are no longer available from Mot Parts.