Looks like they replaced the S&W floor plate with the +2 extension. That would make it a 13rd magazine.
No,
It might be possible to modify the lips on a 20-round 59 series magazine to work but I'm not sure.
I've used 5906 magazines in a 4006 before without a problem.
That is good to know, thank you. I have some pre ban 59xx mags, and know someone who doesn't like his 4006.....