I would not expect to see too much difference in the airspeed readings of the two planes as a result of the pitot tube position difference. If in doubt fly them in formation (not too tight please) and compare readings, also considering the calibration record of the two airspeed indicators, which is more likely to produce an error.
As for the reason for the different angle, with due respect to the other poster, the cam adjustment at the rear spar produces only a small fraction of a degree of angle of incidence change over its full travel. Very important for rigging, but no affect on visible wing angle of incidence. Check instead that the pitot tube mounting tube is securly attached in the suspect aircraft. It is an airfoil shaped tube, which projects down from the wing. It is held in place by screws and riveted tabs. Any of these pieces could be adrift. Also, in 1974, Cessna changed the wing airfoil of the 172. If the two planes span these year, that could be a part of what you are seeing.
Pilot DAR