A previous thread showed a M&P that had been refinished and had the SN from the frame probably accidentally removed by a heavier hand than needed on the buffer. Or on purpose but that doesn't matter for the point of the discussion.

Realizing that no one here can give a lawful ruling my question is if the SN was removed prior to the GCA of '68, before SN's were required, would it be illegal today. As we know some older guns have no serial number from days gone by. If it was removed post GCA '68 then it would be illegal but prior can it be safely be possessed or restamped easily without legal hassles?