Generally earliest original Colts had no markings,/pads/numbers until GM and H&R came into the game then Colt's were marked as on the upper receiver with rectangle pad and number inside a circle and not like aperture beside the windage drum. That one looks like Colt except no rectangle pad. Could be a blem for lack of better term. Probably sub contract replacement part.
Possibly H&R or GM that were different in some instances. I have one that is a raised pad only with two horizontal lines through the rectangle pad only. Always though it was H&R but had it so long now couldn't tell you provenance or really care. Know it was different than all the others I've had and always held on to it.