[Formulabruce]

Hand built engines got their own pallet/bin of Starters, balanced parts, Bolts, manifolds, Pumps and hardware to pull from. The SD was Being built in '72 ( there is one example in existence ). I am willing to bet they ALL had date similar starters due to this process. The engines were in their own "Sub Assembly" area, with their own stock of hardware.
This has been seen in BBC Tonawanda engines that needed some hands on at assembly Time, Also in Flint Built Z-28 engines.
If the SD had a M13 ordered for it, the Transmission could be OVER a YEAR older than the car and be correct for it.
With The originality of the car verifiable, I would stick with your Starter, I am betting its original.
I serviced a '73 SD Trans Am at the Dealership in '78 for overheating, it had only 4,000 miles on it and was Not from New England. Warranty work. First thing I did was look to see if it had a heat shield on the Starter, and I gave the starter numbers to our parts guy to see if it had the correct starter. I distinctly remember him saying it was a "late 71 starter" and should be fine. Heat soak with unleaded fuel and smog equipment was bad in those days. I ended up leaving the starter and Pulled the Pump and changed the divider plate ( we stocked them due to rust ) , and put in a 160 thermostat ( that part was NOT warranty, was my convincing the owner to do it) Quick Flush and out the door.
Don't Discount the The fact that the easiest way to get a engine built may not always follow the "rules" for casting dates or stamped dates.
-- my 2 cents having seen / owned some non correct, BUT Still original cars...