From a catalog circa 1967:
   The PDP-10 includes an extremely powerful processor with 15 index
   registers, 16 accumulators, and 8,192 words of 36-bit core memory, a
   300-character-per-second paper tape reader, a 50-character-per-second
   paper tape punch, a console teleprinter, and a two-level priority
   interrupt subsystem.  PDP-10/20 adds two DECtapes, PDP-10/30 includes
   16,384 words of memory and additional I/O devices.  PDP-10/40 adds an
   extended order code and a memory protection and relocation feature.  And
   PDP-10/50 permits swapping between 32,768 words of memory and fast
   access disk file via the multiplexer/selector channel, and includes
   multiprogramming time-sharing software.