Nope, heath had it there for a reason. It's not my problem part!
Turns out that someone had replaced ALL of the old ECL logic chips with newer ones. What I missed during my first, rather sleepy investigation, was that one of the pins was broken off the flip-flop that precedes the Schmitt trigger. Isn't this what the Derb calls a "Hammy Hambone" moment?
Lo and behold, I had one of the esoteric 95H28 chips in my junkbox...from god knows where.
So, I just plugged it in. Seems to work fine now.