I never looked into what rounds would interchange with the 410, so I can't help with that.
INFO ON MAG DIFFERANCES
2 1/2" (63mm) .410 Factory Mags - Saiga .410 - forum.Saiga-12.com
Yikes is right, I think that's what I paid for the Izzy 10 round mags. I'd like to get a couple of them but I'm paying that much.
Thanks for the link to the 2.5 mag info, that had a lot of good info.
I know what you mean about "back in the day", I got mine for 200 out the door.
The only thing I can think of off the top of my head is to look at how the shells feed, you'll see how it hits that guide above the chamber (and if you try a 2.5 you see it hits the end of it), but maybe if you extend that guide a little it might feed a little better?
Other then that it might need a completely different mag design?