I just pulled mine out and did some looking/measuring.
The "I" in RIFLE is .170" tall.
The "1" in 1A1 is .140" tall.
Both 1's in the 1A1 are the same height, which is different from the height of the "I" in rifle.
I'll say it's a 1A1 based on that.
That said, the "I", and both "1"'s are all just straight lines on the upper receiver. On the lower receiver, I note that the stamping for the serial number is a proper 1 with the curliecue at the top and the bar at the bottom.
NavyShooter