Your 75 is fitted with a D6F4 -01 or -02 dual pick-up coil distributor for manual or automatic Federal or Non Ca. spec Zs.
The timing is advanced 6 deg. (commonly referred to its "phase difference")
During intial start-up while eng. coolant temp is low, thus a WTsw. is closed providing a ground path for the Ign. adv. control relay. The TIU - Transistor Ignition Unit sends advanced ign. signals in relation to engine
WTsw. - Water Temperature Switch opens at it's critical temp. denergizing the control relay, and thus cancelling the advanced ignition timing signals and returning to the 7 degree of initial timing.
Ground out the single red wire going to the Wtsw. located on the engine's
thermostat housing to simulate a cold cond. Check & adj timing for 13 advanced, reconnect WTsw. check & adj. for 7 degree retarded. If not within spec adjust phase diff. internally in the dist at the breaker plate in accordance with your manual. Readjust until both conditions are met