Stumbled across something at a gun show that seemed like an oddball to me today.

In bin at one of the tables was a MecGar marked single-stack magazine that (per my eyeball-o-meter) seemed to be the correct dimensions for a P220 9mm magazine without the back spacer. It wasn't blued, but looked like steel with a light yellowish finish that seemed a little rough to the touch. It came with a flat base and four cuts to accommodate a magazine catch.

It was marked "Sig Sauer", "9mm", "7 rounds" and "European". The seller may have simply counted the witness holes in the side in deciding it was a seven-rounder.

Has anyone seen something like that before?

Modified Sig 225/P magazine with heel release (European) maybe? They only have 7 witness holes

It had the cuts in the body for a push-button mag release, but that might be it. If I go back to that show I'll have to take a 220/9mm mag with me just to see if its shorter.

FWIW, the only engraving or other permanent mark on the mag was the MagTec/"mountain" logo across the bottom of one side.

I never understood by many mags have no indication as to what sort of mag they are on them.

Were the witness holes in a straight line or zig zag?

Straight line.