Well use your head, untill you are sure.. To ATF the reciver is a gun, and a shoe lace is a full auto machine gun..
I have questions I might like to ask, but fear to ask as I have no intent to get you busted.
If you can determin the contry of origin that should be safe. If we knew that we might be able to help better, but I would refain from posting any pics untill you are sure there is no hole where it is shown in the picture, and never was..
If you can run a search of SAR-1 and some how get a close up shot you will see 2 dents one as a 'X" and the other a 'Y'. The Y is where that hole would be.
Yugo's and Chi com don't have nay dents, but the area was fixed either, as no hole was ever drilled there for imported models.
It is true with giood welding skills you could fill and blend in so to the eye no one could tell, but you would never fool x ray, and other machines made to look where the human eye can't. Under professional scrutiny a fill in repair would stand out like a sore thumb..
The risks are not worth it.
If you can see import markings there is a better chance the gun/reciever never had this hole.