Anton,
The story of the Gt is full of mystery!
Those plates must have been put on towards the end of the production line. I don't think each one would have been complied individually. Compliance in those days surely must have been 'riveting a plate onto the car'! Someone, somewhere must know a worker from the factory who can give a definitive answer,
Although the plate only says '1/73' it seems logical that it would go on on the line. But who's to say that it wasn't begun on 31/12/72 and finished on 1/1/73! It's a minor point I guess as we can definitely say production ceased in January 1973.
BTW, back on original topic, I have 3. 2 are roadworthy - both 1/73 cars and 1 currently being restored (very slowly!) which is a 12/71 and registered here in 04/72.