this is from an E-mail attachment from to me by Sid Woodcock one of the originators pf "DETONICS" It was writen by "Chuck" his ex-armorer. Hope it is useful because in my search I was told many different things about ID'ing
A combat master.

"The twenty
thousand serial number range
was used to note the carbon steel guns from the new casting tool. We
end the first run of cast frames
somewhere in the 9000s, then the stainless Combat Master frames began at
CR10000. Note the R, the R
appears in all stainless Detonics pistols. CRM-Scoremaster,
Servicemaster and Janus pistols. The
earlier 9000 or so guns didn't have a letter prefix until the early
4000s, I think. The letter 'I' was used
then changed to a 'D'. The letter prefix was required when ATF figured
out that Cal Foster was
machining more than one companies frames and we had to make them
different somehow."