retarmyaviator
US Veteran
It would not hurt as a courtesy to the FFL dealer to let him know so that he can correct his records if he so desires.
Two answers…
If the FFL used the "assembly number" as the SN and it was recorded as such, not sure there is anything to do about it. If you sell it use the real SN.
The other stampings are various factory internal codes from assemblers, fitters and inspectors.