The other day I was looking at some M16A2's that were in use by a DOJ entity and I saw something strange on a few of them: The rear left side of the area under the buffer tube had M16A1 reinforcements instead of the A2 ones.
The weapons came to them from the DoD and were Colt roll marked M16A2's with the USG property stamping. I did not look at the right side to see if it were done there also, but it was an interesting thing to see. The receivers were in the very low 8 million range.

Anyone ever see this before?

Yes, common event.

From what I was told, Colt used up older A1 lowers where they could, even on A2 rifles.  I have a commercial style A2 magwell demil on an A1 style lower 8mil range and another commercial A2 what's only about 8000 units off with an A2 lower.  Not government property marked, I know, but an example nonetheless.