Will my plain old 228 mags, work in the new SIG M11-a1?

Thanks,

Yes, it will.
The other way around, 229-1 mag in P228, will not, however.

Thanks "Q"

I just might have to pick it up then.

Old 13rnd P228, P229 mags will work, also P226 9mm mags will work as well. They will wiggle left to right some though but they will work.

P229-1 mags will be ideal though.

FWIW I've had a couple of feed problems when using original mags in a 229-1 magwell. I don't fully trust that setup. It may just be the ones I used, but I'd test before any serious reliance.