seamusTX wrote:I'm just wondering if it is legal to add such markings on one's own.
IANAL, but Federal law requires serial numbering of the receiver of the firearm. That means (for example) the frame of a 1911, the frame of an S&W revolver, the lower receiver of an AR15, the upper receiver of a FAL, etc.
AFAIK, Federal law is silent on engraving or numbering things other than the receiver that's required to be numbered. In fact, things like dates are often the subject of engraving on guns and gun parts, so I would have NO problem whatsoever engraving additional numbers on other parts of a firearm such as the grips.
In fact, so long as you do nothing to obscure, impair, alter, or conceal the original, factory-assigned serial number, I don't think there's any law to prevent you from engraving random numbers all over the rest of the receiver itself as decoration.