Guessing here a bit...........The diode that failed in the rec module failure mode was probably a short/low low resistance fired pn junction. The tach signal wire (gray) is jumpered on the terminal block (see below) such that it is wired to the side (two sides available, Both AC, yellow and yellow - gray [yours]) of the rectifier bridge where the shorted diode occurred (you had a 50/50 chance)........
This shorted diode was on the "-" side (another 50/50 chance) of the bridge, this shorted diode beings it was wired on the "-" terminal of the bridge then pulled the tach's signal to ground'ish.(Tach goes dead, signal wire is shorted to ground via shorted diode to ground)
Either yellow or yellow-gray wire however should work, esp now that the shorted diode (bad rec) is removed...The tach's signal (derived from the charge winding AC pulses) should be the same on either yellow or yellow-gray.......
