The problem with the RD boards is they require the same obsolete opto. RD had a batch of opto's custom built for him -- the only place you can get replacements is from RD, the manufacturer will not sell to anybody else. Instead of fixing the problem with the opto high current loading, he duplicated the same problem that existed with the old Williams optos and eventually they will suffer the same fate - you'll be back to square one again.

I am skeptical of the existence of NOS QVE11233.0086 optos. There was one electronic parts distributor (forgot their name) that magically keeps getting thousands more of them - they currently show 55K+ in stock. Considering that these optos were custom screened for Williams many years ago - I have sincere doubts that thousands more keep showing up long after they were no longer being made. MrPinball in Australia may have originals ( I do not know) but do beware of many to be found out there through various distributors are not originals. These are non-screened parts that might work... might not. The good thing about MrPinball - he's located in same country as you so shipping shouldn't be too bad.

Marco Specialties does advertise a replacement - Optek OPB804 if I remember correctly. These are very close to the originals (similar to QVE11233 with no suffix) but they aren't screened to the .0086 specs. Often works as replacement but not guaranteed according to vendor specs. Went through an evaluation quite awhile ago with the similar Vishay TCST1103 to see why these don't always work. If I remember correctly, there was one resistor you can change to make these work but I cannot remember which it was. The resistor loading choice for sensing voltages from the optos was too small in value resulting in excessive current draw. Such a high current load that many existing optos can't provide enough current drive which results in an excessive voltage drop across the opto - hence the special ordered version. I'll see if I can find resistor info again. If I remember correctly, opto used 12V at collector input - emitter output was fed to a voltage divider which then compared the output value to +5V. But it has been too long - I don't remember.

Ingo actually designed his board to work with existing off-the-shelf optos such as the TST1103 and OPB804. To get around the high current issue - Ingo redesigned his board correctly. He essentially 'redrives' the opto output so that the output line is compatible with the existing current loads and comparator circuitry.

Yes - I am skeptical of these. There seem to be too many of these floating out there on the internet with at least one vendor that claims to have more than 55000 of them available. I never did say who. Your claim of a few hundred seems to fit more in line with what I would expect to find out there. But thousands at a time - no.