Surely the 2100S has user-writeable microcode? The
front panel
says "Microprogrammable Systems Computer":
See the rest of my 2nd paragraph previous :)
The 2100S, from the factory, had sockets where you could plug in user burned
microcode chips. The 2100A did not. However, if you ordered addon firmware
for the 2100A, you could get the socketed version of the cards.
If you mean user writeable microcode as in not having to burn your own
chips, either the 2100A or 2100S could have that option board installed.
Separately ordered and purchased option board, not to be confused with the
open rom sockets on A1/A2.
Jay