I have noticed the latching current requirement of SCRs is all over the place, even in the name brand parts like on semi it varies lot to lot and even piece to piece. I tested four different brands/part numbers and go figure the cheapest Chinese brand from LCSC has the best latching characteristics out of all the ones I tested. I had one lot of on semi SCRs that wouldnt latch for crap with LEDs unless i really stiffened up the resistors to about 270R-330R. Still was fine for incandescent.
I assume the means the original lamp boards are also going to show some variance on latching current and whether or not you get full latching with an add on board.
Once I run out of the current PCB layout I believe am switching the lamp board to use all C106 SCRs and no TO92 packaged ones. The larger packaged C106 SCR seem to have better latching characteristics even tho the data sheet says it shouldn't.
For stern MPU200 games the lowest decoder positions you need about 470ohm load resistor @6v or else you have a good chance some LEDs will not latch the SCRs. For Bally -35 games its about 680ohm on the lowest decoder positions.
Also minor differences in CPU board clock speed will effect latching characteristics too. Its why MPU200 games will always be more likely to have flickering LEDs than Bally games. The faster the CPU clock, the closer they update the feature lamps to zero volts and not enough current flows to latch the SCR.