Milwroad-

I cannot speak to post-63 manufactured M70s with any confidence.  On the pre-64 M70s the routine practice was to electropencil the serial number on the underside of the bolt at the time the rifle was proofed.  Same time that the WP proof marks were put on the receiver and barrel.  Since by this point the rifle was fully finished and assembled, these markings (proofs and bolt number) are rough and not polished over.

On pre-64s the bolt serial numbers were often quite faint and I’ve seen many where the number is no longer legible in its entirety, but unless the bolt body was polished at some point you can pretty much always tell that a number was there at one time.

The only thing I’d want to do on your gun is to check the headspace before firing, just in case the bolt is a replacement.  Better safe than blown up… 

Sorry not to be of help,

Lou

The usual reason parts were numbered was because some hand fitting or hand selection was involved in assembly; the manufacturing methods of the time just weren’t precise enough to guarantee that every single part fit together perfectly with other parts.  Modern computer-controlled machine tools & laser measuring devices have made this unnecessary, & that, I think, is why the late ’70 bolts weren’t numbered–they didn’t need to be.

The most thoroughly numbered rifle I’ve ever examined is my 1952 Russian SKS–about the only un-numbered parts on it are screws & pins!  Don’t know if this reflects the crudeness of Russian manufacturing methods or the famously obsessive Russian inspectors, who made life hell for US gun makers who accepted (& usually came to regret) Russian arms contracts.