My parcel from Greg Cote came today, way ahead of schedule.

Pics below.
I was surprised to see that the 10rd Mec-Gar mag has a two-piece body; metal on top, plastic on bottom. Not a converted 14rd S&W clone. The 10rd observation hole is the same but the Mec-Gar has a 5rd hole also. (The S&W has a 14rd hole). The Mec-Gar has an extra hole on the front of the metal body, just below a
"Made In Italy" stamp.
The Mec-Gar follower is shaped different and it is a bee-yitch to get the first round in. After that it gets a bit easier.
The Mec-Gar comes with a golf 'T' like plastic depressor to facilitate disassembly of the magazine. Nice touch.
As soon as I took it out of the ox, I tried it in my S&W 659. It went in nicely...but would not lock in place...It would fall out...
I checked the mag button; it was in the depressed position when it should have been in the locked position. I took the slide off and verified that the mag lips weren't the problem, it was the 'foot' of the mag plastic base (last pic). By smacking the base of the mag hard I could get the mag button to seat correctly. I've determined that it is a very small bit of clearance in the mag button slot that is keeping the mag from seating (until the button finger is jolted into the mag slot).
A quick and dirty fix would be to file the magazine body cutout so the finger would engage. I don't like that idea. Looking closely at the bottom of the plastic mag base, the 'step', I can see a sort of 'pad' where the base contacts the grip strap. I'm going to take about .005" off the 'pad' to allow the mag to engage the mag button.
Now I just have to figure out how to do it...