In all likelihood, a former U.S. Dept. of Energy Colt M16A1. Not particularly rare as many such marked M16A1s were issued for security use on the Alaskan Pipeline; and at U.S. nuclear power plants all across the country where they sat locked up and were never used. They used to come up for sale fairly often 15 or so years ago.
If you haven't already done so, you could do a FOIA request to try to pin down transfer dates.
As you say, something for a collector.
MHO, YMMV, etc. Be well. 2A