This is the one I gave Mark Humphreville at Picatinny Arsenal.
SEARCH: Lutz on SAR site and click on the pdf manuals for the M16A1E1.

http://www.smallarmsoftheworld.com/search/pdfdmp.cfm?docid=888.pdf

This SAR is a treasure trove of information, but I could not find my M16A2 interview from about 1995 or so...
Hello and thanks Mr. Dan Shea!

The hole in the back was an M16A1E1 test sample characteristic (as illustrated in my sketches) and was originally a feature of the Colt LMG that had brought the concept to us, but after the Operational Test we changed it to the left side because the troops had issues with it as well as being able to tweak a full 800M out of the dial; and that allowed us to replace the rarely used 800M M16A1E1 flip aperture for the M16A2's more useful and practical 200M ghost ring.

Such uppers are extremely rare outside of a few military museums because it is likely Colt made less than 50.  And those uppers would have been made from highly modified M16A1 upper forgings.

All the test samples (about 40 each) had M16A1 teardrop forward assist plungers.  The round plungers came later.

The hole at the rear of the receiver looks larger than what we tested as best as I can recall.
It does look a lot like my sketch in the manual, but I probably took some artistic license there to make the numbers look bigger.
Also, the side view does not show a polished-over weld line as did the prototypes we tested. This was a block of aluminum which later was machined for the rear sight parts.
You can see that in this photo;


I think you have an A2 upper that has been modified with the rear hole.
Note the integral case deflector.
It also has the A2 windage drum, not the square-cornered M16A1E1 type.
Also you have the M16A2 "0-200" peep.

What are the range numbers on the elevation dial?

Yes, MGN's.
I found a pdf copy on my hard drive, but don't know how to make it available on-line to you guys short of making each page a photograph as I here with the cover: