You can file a Freedom of Information Act request with NFA Branch. It's free and it usually takes months to receive, but you do find out (1.) the original barrel length and overall length; (2.) how many owners it has had (though the names are redacted) and whether or not it was ever used by law enforcement; and 3.) the day the Form 2 was filed, so you can throw a birthday party every year.

As far as your question above: The were only done in blocks within contracts -- if an agency ordered 500 in a certain configuration, they built 500. But they are not necessarily sequential: Colt made up the lowers in huge batches, and drew them at random to fulfill orders. And there were usually a few "stray" non-sequential receivers in each contract lot.

HTH.

Interesting they are popping up NIB in Europe now. I'm guessing they were exported years ago and said in a warehouse somewhere. Under U.S. law, once exported, they can only be re-imported for possession by LE/military, and they already have enough here.

FWIW, my Colt M16A1 is a 9,3xx,xxx SN, so not all of that SN range went to Israel (I've seen quite a few others here in that SN range, too).

Ha! I looked for years for a correct Colt bayo made at the same time as my M16A1. I finally found one ... in Europe! (Well, reimported from somewhere in Europe.)

OK, a few bits of info I've learned:

--The 9-million SN range was designated for law enforcement and for export. All of the receivers were A1 forgings (there are examples of receivers that Colt replaced post-1986 and moved the SNs over to post-'86 receivers, which makes them something of a gray area as far as 922(o) goes).

--While the earliest were roll marked just "M16" before going to a universal "M16A1" roll mark, all were built in A1 configuration. I have never heard of a 9-mil gun lacking a forward assist, etc.

--The three factory configurations of the 9-mil guns prior to May 19, 1986 that I have documented are: 14.5" carbine; 20" (thin) barrel rifle; and 20" HBAR rifle.

If someone can document a transferable 9-mil M16A1 that left the factory in any other configuration, I would be thrilled to add it to my database and knowledge base!

YMMV.