I have a TOS SN40C. The A, B, C designation refers to spindle bore size. A’s are the older machines; C’s the newer. TOS was continually improving and refining the machines through the years so there are other changes. My C machine has about 2 1/8 to 2 3/16 spindle bore. You can probably find the other bores on the Lathes.co.uk site.
The serial number will tell you the year of the machine. On my machine, the serial number format is #### ## ## #### with the third set of numbers reflecting the year. In my case, it reads #### ## 95 #### indicating a 1995 machine.
My machine was in good condition, but I went through it anyway. I disassembled the apron and opened up the feedbox and headstock. I also made a few modifications to mine regarding the machine bases and adding a DRO in such a way that it wouldn’t interfere with the machine feed kick-outs. I’m mentioning this because I took quite a few pictures. If you need any insights, maybe I can help.