Joy! It's fixed. The culprits were Q13 and Q14 transistors on the SDB.
Thanks for all the suggestions.

I'm glad you fixed the problem, but can someone out there explain to me how a bad Q13 / Q14 would fire the wrong coil ?   I can't begin to understand it.  If it makes sense, I would have certainly suggested SCR replacement right at the start !

Quoted from mrniceguy

I'm glad you fixed the problem, but can someone out there explain to me how a bad Q13 / Q14 would fire the wrong coil ?   I can't begin to understand it.  If it makes sense, I would have certainly suggested SCR replacement right at the start !

i'm confused a bit on this too mrniceguy. In the end I was looking at multiple areas for a possible cause for the problem. The expander board seemed ok, but was buzzing during lamp test mode. I resoldered all the pins on the SEB, and checked the harness plug for problems. Still buzzed. I then swapped out the power supply/rectifier board with a spare I had, and then the buzzing turned into the healthy "clicking" I was looking for. Still did not fix the misfiring of coils during game play or solenoid test mode. Swapped out the lamp driver, to eliminate the possibility of the 555 lamp next to the SEB from malfunctioning, but still no joy. I was wondering about a possible bad diode on one of the coils associated with the solenoid expander, when I decided to just replace the Q13, and Q14 transistors on the SDB. This finally fixed it.......but I'm not sure how. To be honest, I unscientifically replaced U10 and U11 at the same time as the transistor swap, so maybe my problem was there?

Whatever it was it's gone.......for now.