Thank you for the compliment, DaveP1!
Yeah, M16s in general have been an obsession for me since I was a kid (first shot one in the '60s), and in particular, the 9-million serial number range ever since I bought one of those in 1994.
To the OP:
I have a Registry of a couple thousand 9-mil M16s. The earliest SN with the "M16A1" roll mark (the very first ones were roll marked "M16") I have seen is 9103617. To give you an idea of a production date, 91608xx was Form 2'd by Colt in May 1977.
But remember what I said about Colt not producing them in numerical order. For instance, an FOIA request shows a pallet of 72 with SNs of 9268636 through 9268671 (yes, they skipped a lot) was Form 2'd on April 4, 1979.
Yet mine, 9309864, was Form 2'd by Colt on March 7, 1978.
The only way you can find out the exact date of yours is to file a Freedom Of Information Act (FOIA) request, once you are the registered owner. It can take months for ATF to respond, but the process is free, and then you have it in writing. (You also have clues to the original configuration, from BBL and OA lengths.)
About all you can be assured of without an FOIA report is that it was most probably made in the late 1970's.
Hope that is of some help.