I should have listened... tomorrow I'll clip on pin 7 of the LM2903 like you said. I'll see if I can log at 50kHz to get good resolution. At the very least, 36kHz.
So I'm guessing I was measuring the speed sensor (diff?) that feeds the speedometer? The signal is definitely speed dependent.
1993 245 Classic
1992 245+T (DD) Build thread here.
1972 P1800E Chassis number 39412 (Doesn't exist according to Volvo)