1903A3 &1903A4 Rear Sight Witness Marks?

Both of my 1903A4 rifles have this vertical line (see 1903A4 picture) on the rear, right side of the receiver bridge. I thought the line was stamped on after the 1903A3 rear sight was targeted. (see 1903A3 picture) If that's the case, what's the line doing on a 1903A4 receiver? Could the line on a 1903A4 indicated the rifle failed 1903A4 inspection and was issued as a 1903A3?

Baffeled (again)
Chuck

i have several new in package sights with a witness mark on them, as for the A4? well, id bet that line was made before the lettering, and the holes drilled.
and part of the proccess of the making of an A3 that was included in the A4 as well.
just like the stripper clip guide, cant be used on the A4, so why have one on the receiver???it was just part of the making of the reciever.