I ran two 6-rounds mags of SD ammo through the gun after I bought it. Reloaded the mag "by hand". Then I bought a Maglula UpLULA MAG loader.
I was waiting for the 8-round mags to wring it out more. I went out to the Nat'l Forest to shoot today. First I ran out the SD ammo that was already in
the gun. The 8-rd mag in the gun. And changed to the 6-rd mag. No problems. And switched to practice ammo.

Two things developed relative to the magazines (the gun was fine).

First, I used the loader on an 8-rd mag. The metal baseplate has a "pin" holding it in place. The plastic "finger" extension extends below the metal baseplate
and all around the metal magazine. And, it is free to slide up on the mag. All the way to the top and off the mag if you want.

In order to seat the mag in the gun requires a REALLY HARD SLAP to the mag with the heel of your hand, at the back of the mag where the metal base is
closest to the bottom of the plastic "finger surround". If you don't DO THIS the mag WON'T seat. It WON'T SEAT. It is easy to "miss" this.

You can seat the 8-rd mag with "normal" pressure by placing your thumb inside the plastic and pushing directly on the metal baseplate. Not a "reload" method.
And, I guess I'm supposed to "glue" the plastic finger extension to the metal mag ?

Second, I started to load the 6-rd mag. After two rounds, the baseplate flew off, and part of the spring came out. It turns out the baseplate is plastic and has
internal slots that slide over metal "rails" formed at the bottom of the mag. There is NOTHING to retain the baseplate, and it is easy to slide it forward
(and off), even when the mag is fully loaded. It has a hole in the center of the plate in the same place as the metal 8-rd baseplate, but no retaining "pin".

When reloading the 6-rd mag I had to place a finger under the UpLULA loader to keep the baseplate in place for EACH round. Failure to remember that caused
the baseplate to fly off because of the "loader angle". I'm ashamed to admit how many times I forgot and that happened.

Questions:
Other P290 owners: Do your 6-rd mags have this system of a plastic baseplate "just sliding on" with no retention method ?
Other Siggies: Do any other Sig mags "operate" like these two ?

I'm wondering if my 6-rounder "missed" an assembly operation ?