Looks regulation to me. Got a photo of the "squared" one?
Edit: Took me a second look. Yes the handle base seems squared on Pic 1. I have never seen, heard, or read of its significance. The struck out serial number may indicate that the bolt was recycled or the rifle refurbished. It looks older. As a guess, this rectangular pattern may be contemporary to the hexagonal Mosin receiver, before those parts were rounded in the first Soviet edition.