The release notes ("RELEAS.NOT" on diskette #4) of the DECnet/DOS v1.1 on
bitsavers
http://www.bitsavers.org/bits/DEC/decnet/
has a bunch of comments about the Rainbow.
Was there a separate release for the Rainbow??
You're right that the RX50 diskettes in the Rainbow weren't PC compatible, so it
could simply be different media.
Bob