mjlan is correct on the partial vin. 1A is a 1971 Atlanta built vehicle. I sure hated it when I heard Ford was closing that plant. I had a 70 Torino GT and three Gran Torino Sport's that were assembled there. All well built cars.
David the metal tag to paper transition was officially listed as 2/1/73. It was a corporate change for all US plants. With the sheer number of vehicles involved there was a lot of variance in what you saw on the actual vehicle builds as the plants worked down old stock of engines.
delawarebill, not sure what engine/transmission you have, but the Mustang Tek site David listed is an excellent reference source as far checking ID/Vin numbers and where they can be found. I have found that 99.9% of their info is spot on. Anything that was incorrect was minor and easily checked against other sources.