There is also a 9mm/38super mag that has an...a...shucks can't remember the name for an indentation in sheet metal -anyway there is an indentation running the length of the magazine about one third of the way back from the curved front on both sides. This was intended to help keep the cartridges stacked straight but was dropped; it seems the standard width works fine.

There is a mark on the base but you can't see it...

Anybody know? ;-)