The info on Wiki seems to be corroborated by this:

http://oldcomputers.net/dec-rainbow-100.html

First Rom, HD not supported = Rainbow 100A
Second Rom, HD supported but not factory installed = Rainbow 100B
Second Rom, HD supported and factory installed = Rainbow 100+

Robert

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