Had to go to the shop and look at my '72 Duster 340 and it is indeed for a clutch pedal lockout switch which goes to the starter relay on the inner fender.
I checked the firewall of my Demon this morning
Sorry for the bad picture but it's not easy to slip a telephone among this pile of wires, cables and pipes
Tjpatte, Dana67dart and WymanV are right : thru this hole goes the green wire for the clutch safety and its grommet.
Your firewall seems to come from a Demon which had a manual gearbox.
Thanks to this picture, I noticed that my handbrake cable grommet is incorrectly positioned