Bill Kibler, the PinWiz, is right: "credit unit" is the industry term,
along with "replay unit."  He is also right in that bonus counters also
make good up-down counters.

Another item to consider from a salvaged pinball machine is the Ball
Count unit.  This is yet another toothed wheel with two coils.  On most
machines it will count only up to five, but on "Add-A-Ball" pinballs it
will go to 10.  Like the Bonus unit, it has no window but does have
switch contacts to light up 5 or 10 bulbs to show plays remaining, if
you want to add such a display to a piano.

Just like some player instruments, there are pinball machines that must
sadly be considered as beyond repair and restoration (as when missing
the unique art glass), and used as organ donors to restore those more
fortunate.  There are plenty of parts out there if you know where to
look and whom to ask.

Good hunting, Mike Knudsen