The M9313 is an intelligent followon to the dumb M9312/M9302.
You can use the three modules interchangably, but modern DEC
diagnostics can do a better job of exercising and testing your
bus if they can detect an M9313 at the end of it.  Unibus systems
under contract will all have been silently upgraded to M9313's
by this time, unless your field service engineer is a putz.