Just a misunderstanding, when you stated, you thought it was aftermarket engraving, I thought you meant the receiver, but I see now you just meant the serial number.
But never waste an opportunity for good photos....

This is a 1966 M700 I sold a few years back, it was NIB. Hard to tell from the OP pictures, but the engraving does look very similar, if not the same, at least on the fences...
![[Linked Image from i.imgur.com]](https://i.imgur.com/eRs8q2i.jpeg)
![[Linked Image from i.imgur.com]](https://i.imgur.com/lMu386u.jpeg)
Clarification.
Whilst I am unable to view the images you have posted, I will only add that my opinion was based solely on the KB 138497,on trigger guard tang, unheard of with letter prefix and
having been re engraved and not looking correct. I am not in any way questioning the integrity of the gun.
Regards