Hard to say anything for certain witout actually examining the gun, but I don't think it's a weld at all.
Firstly, that is a very strange place for the frame to suffer a stress crack.
Secondly, that is not the way a crack would be welded if it was done properly.
Third, there is no evidence in the photo to suggest that a weld has been done and finished off.
The metal on either side of the "weld" appears to be original, and I doubt that would be the case if there had been a crack which had been repaired by welding.