Murano glass crack or cooling mark?

Hi. I should start off by saying that I know very little about art glass. I recently picked up what I think is a 50s/60s era Murano piece in the shape of a clamshell. It has two bases, so it can stand vertically or tilted forward. When I look down into the mouth of the glass (I hope that's the correct terminology), there is a line that to me looks like a crack, but the line is entirely under the surface. I don't feel anything different when I rub my finger over that section than anywhere else on the piece.

Since it's under the surface, I'm wondering if that is just some kind of cooling mark from when the piece was made. Or is it actually a crack or maybe a crack that's been repaired?

thanks, Mark