These tapes where used in a number of machine such as
9825, 9831, 9835, 9845,
 9915, and 85A&B.  There was also at least 1 external tape drive that used these
 tapes not to mention DEC made a drive for them as well.
 If you hope to recover the data off the tapes you would probably need access to
 the system they where written on as the data format is not the same on all
 machines.
 I can't think of any of the 68K 9000s that used DC100A tapes by then almost
 everything was diskettes 8", 5.25, and 3.5 where all supported however some of
 the early software for 200s was quite restricted in the formats supported.
 The DC100A has the same arrangement inside as a DC-300 just smaller.  I have
 had some success replacing rotted drive belts with PLASTIBANDS.
 Paul. 
Aagh, I forgot about the HP85 some instrument programs where written for those.
So you have to check the part numbers of the tape and look in the service manuals to see
where the tapes are for.
The 9875A and 9877A are external tape drives for the DC100A tapes almost unobtainable like
the HP 9831A which is also very rare (I know just 3 including mine) .
The only 9875A and 9877A I know are at both the HP Museums.
-Rik