[Simh] RB09 == RD10

[Bob Supnik]

Max Burnet gave me a pointer from some old price lists, showing that the 
RD10 had very similar specs to the RB09. The RC10 manual confirms it - 
same BCD addressing of tracks and sectors, same number of tracks, same 
sectors per track, same words per sector (32 x 36b for the RD10, 64 x 
18b for the RB09). So the "RD10s" on some PDP-9s in the services listing 
are actually RB09s, at least at the drive level.

I still don't know what an RC09 is. Alternate name for an RB09? 
Half-sized variant? Another mystery is who made the actual drive 
mechanism. It precedes DEC's first internally designed fixed head disk, 
the RF09/RS09, by two years.

/Bob