For years, I put out Ball Bowlers for a quarter a play, and still have a couple out for that price.
But the last few years, I made them so they're 2 quarters per play. How I did this is by splicing
a 1950's bowler score reel unit between the coin switch and Coin Relay (or Start Relay on some
pins). I made the run-out switch (which opens only when on zero) so that it opens every other
step up by grooving in 4 more notches. So every other step up the set of contacts open, and
every other they close. The coin switch energizes the Score Reel's coil, and the Coin Relay is
also tied into that circuitry, but it's only completed when the (use to be) run-out switch is closed.
So being closed every other step up, it takes 2 coins for the Coin Relay to energize.
Those old Score Reels are built strong. I've been doing it this way for the last 12 or 15 years,
and never had a problem. The converted Score Reel on the very first bowler along with the
others are still doing their job problem free..

I've also tried it a couple other ways with a single coil Step Up Unit and also a Latch/Trip Relay,
and the drum unit is the most sound, plus I have a ton of them around..The trickiest part is
grooving in the additional 4 notches, but with an (inexpensive) Rotary Tool, its quick and a piece
of cake..

Yes, the only thing that can be messed up is the center hub that has the run-out
groove and 9th position lobe, and even though I made the first ones generically
(used a small rat tail file), I haven't messed one up yet. I now use a Rotary Tool
which makes it much more precise..

All including the very first one that I used the Drum Unit on hasn't failed yet, and it's been quite
an few years. On the ball bowlers, with a gone through drum unit, the pulse from the coin switch
has no problem fully energizing the coil. I do have a few EM pinballs on (selected) locations, but
I kept them on a quarter, but the coin switch pulse I'm sure would be the same. The only thing
that might have to be change is the 50V Drum Unit's coil (depending).

Yup. I've done it this way, but because I have so many extra parts ball bowler drum units,
I went with that route. Those 50s drum units are built, and actually the run-off switch is
already wired to the coil the way it needs to be..
I have found those Interlock Relays to be handy when doing other types of projects also..