Everything I’ve checked has been in line with electrical diagrams… not checked everything, but everything I have has been correct.
The 54/55 were correctly in 49/47.
Spindle activates with 51/53 removed.
Yep, measured across the board terminals and at the wires (in case I was being stupid!).
Not tested against ground.

Disconnected 54 & 55 from Sprint and fitted a 10k linear potentiometer;
Spindle fires up as machine boots up (incl. with SGR removed).
@10k, spindle was running at approx 1k rpm (by eye/estimated)
@9k & less, spindle increases in rpm rapidly
I.e. NOT 10k = 250rpm / 5k = 2000rpm / 0k = 4000rpm
More like, 10k = 1000rpm / 9k = 2500rpm / 8k = 4000rpm

Increasingly looking like both the Sprint drive and Baldor board are both dead..?
Sprint boards seem readily available, but anyone know of any Baldor boards? (Doubtful I know…)
Other options…?