I can't help with the location, but in that timeframe IHC used a low air pressure switch that looked like an old Ford oil pressure switch, had black plastic "cone shaped" top with one terminal and took a special socket to remove (but could use a 1 1/16 wrench). I'd look on the firewall on both sides (inside and engine side). Alternatively, you can trace the air plumbing from the "wet tank" one line should go to the air gov and another line should go to the low air pressure switch.