"I may be wrong, but the mag well looks a bit slimmer. Is that possible, or am I just getting old?"

Interesting comment.
What you might have is a single stacker action inwhich case the mag will only hold 6 ????? 8 rounds.
This being the case ,the front of the magazine well will measure aprox 18 mm and the rear will measure 24 mm

Did you read my previous post on this subject?

Al