I guess I was trying to put myself back in the 1980's "be as compact as possible" mindset.
Though my Google-fu was not doing well last night, I did finally find the manual for this product:
https://colorcomputerarchive.com/repo/D ... Manual.pdfAnd, on page 30, the schematic is there.
As you surmised, A0-A3 is the output of the 7493 counter.
The '93 is clocked from Horizonal Sync, via an inverter. So, increment once per line
The '73 is still confusing me, though.
DA0 will toggle during the screen addressing. It's then inverted, so first char location would be low to high, which means the JK is clocked, J is 1, K=0, meaning Q would go high as the DA0 returns to 0, I assume after putting char position 511 on the screen.
Thought it says Q on the schematic, pin 13 is actually !Q, so Q1 would go low.
Low would then enable the RP signal (!RP, but inverted) going into the '125, which then would put low at the R pins on the '93.
counter counts.
When !RP goes low, RP would be high, and the counter resets.
I guess the '73 was to create an initial !RP signal (the datasheet notes that RP does not trigger until 12 lines have been output) to reset the counter at the beginning of a frame, but it seems like it would have been just as easy to use !RP & !FS as the input to the reset. I must be missing something....
Jim