I realize you wanted to keep the look of the original design but you probably should have re-worked the 5V section using modern day switching or buck regulator technology to eliminate the ceramic power resistor and the large filter caps.

What I didn't originally realize is that comrep "Repro" board is mostly a "dummy board" to fill the original foot print of the Bally SIA. Only the components you see in the first photo are necessary to make it function. The rest of the board is just for "looks" and serves no function.

I realize the intent but its probably not going to fool too many. Without having the original silk and proper logos, it seems pointless to me to go to all the trouble and a bigger board just to have something that resembles the original analog SIA board. Either of the 2 plug and play subs already out there work just fine.

At least to my ears, the HT8955 design floating around out there is true to the original sounds of the SAD IC and Bally SIA board. And so is the simple MN3205 chip replacement approach. In fact, for me, I prefer using the MN3208 which has half the buckets. You get the same required reverb effect but you can actually understand the words being spoken. I stumbled on this since most of the MN3205's I received from China were duds so I thought I would give the 08 a try.

The interconnect harness is documented in the manual which is online. I think if you search S&T, there were photos posted here before.

Its basically two shielded cables to feed line audio in and out.

As I recall, you are correct since it has the two speaker wires at the end of the connector.