It certainly appears that it was a full-auto M16A1 lower originally. It was most likely repurposed to be used for a M16A2. It looks like the AUTO was machined down and BURST was stamped in. Note the recess where AUTO would have been. Additionally, the font used for BURST on the left side is the same as SAFE, SEMI, and BURST on the right side. This font differs from the SAFE and SEMI on the left side. The A1 lower would have not had anything stamped on the ride side. The selector was changed out as well. M16A1s had a no-tick selector. You can see where the it was marked M16A1. The "1" is just visible just in the front part of where the "2" was stamped. Besides, the "2" is a different font.