I picked up a 6906 yesterday. (I really like the trigger pull on this one).

The mag it came with has a single mag release slot on right side.
2 new mags came from CDNN yesterday and they have a slot on both left and right sides.

I noted the following:
With the new magazines, the slide locks back with empty mag, but when I depress the slide release with my thumb the slide will not release. The slide releases fine with the older magazine.

I haven't fired the weapon yet.

I was just thinking. I wonder if it's because the springs are so tight on the new mags??? Never had that happen before.

I believe you are right. After some use, it will not be so difficult for you to press the follower down.

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Ret. LE, FA Instr, S&W Armorer