I just found this

Model B Rom Images

with the Rainbow 100 Model B ROM images. Wiki mentions that instead of three, it only uses two chips for the ROM, this is consistent with my mainboard.
The third EPROM must then be the character generator which I'll try to dump tonight ... I'll bring out my old DELA Eprommer II ;-)

Robert

NCR DMV- DEC Rainbow- Siemens PCD- ITT 3030-Oly People- Acorn A5000- Olivetti M20