I've traced out more of the circuit, it certainly is complicated, but there are similar circuits around the U1/U2/U3 regulators.
Here are some further associations between the power supplies, comparators and digit parts of the circuit.
U1 connects to Q2 which also has feed from U32 pin 10 (7407 driver, nb input pin 11 is connected to U24 pin 6, which is 74LS74 output Q1 bar), which drives Q1, Q7 and TP1. Q7 connects to R40 which feeds into U14 pin 6 (-ve input to comparator 2). The output of comparator 2 (pin 2) is fed to U24 (74LS74) pin 13 (CLR2). Output Q2 (pin 9) is connected to U31 pin 31 (8155 PB2).
U2 connects to Q4 which also has feed from U32 pin 12 (7407 driver, nb input pin 13 is connected to U24 pin 8, which is 74LS74 output Q2 bar), which drives Q3, Q8 and TP2. Q8 connects to R41 which feeds into U14 pin 4 (-ve input to comparator 1). The output of comparator 1 (pin 1) is fed to U24 pin 1 CLR1. Output Q1 (pin 5) is connected to U31 pin 32 (8155 PB3).
U3 connects to Q6 which drives Q9, Q11 and TP3. Q6 output connects to Q9 base via R39, Q9 is also driven from U17 pin 1 (MC1489 - not traced further). Q11 connects to R42 which feeds into U14 pin 10 (-ve input to comparator 4). The output of comparator 4 (pin 13) is fed to U25 (74LS74) pin 1 CLR1. Output Q1 (pin 5) is connected to U31 pin 30 (8155 PB1).
On to some further tracing, and some more captures of oscilloscope traces of the comparator outputs.
Regards,
John