The takedown pin/safety with "holes", that's the way the early ones were. Don't remember if I ever heard as to why. As to the early flatside receivers with the Colt Ar-15 Rampant Colt markings, I've seen several examples. Normally you'd see stamped under that USM16A1 with the serial prefixed by SER. under that. In a batch of 200 from ANAD, these were DRMO guns to a LE agency, right out of the wraps there were several of these. One had XM177E1 but did not have the stock/barrel for that, it was an 20" pencil barrel A1 with an A2 stock.
I know is happens that Colt will use guns for demo models, armorer use for teaching, trade shows, etc and then down the road sell those. Wouldn't mind finding a Port Firing M16 but I suspect all of those disappeared to the winds....