The mags for the MAS 45 or Model 45 Mauser .22 will work in the B series rifles (MS420B, MM410B, MS350B) and I think they will also work in the MS420. I've seen quite a few of these on gunbroker at modest prices.

You might also contact Classic Arms in S. Africa - they sometimes have prewar Mauser parts. (www.classicarms.co.za)

Final thought is Hoosier Gun Works (www.hoosiergunworks.com). They usually have a wide selection of magazines for German .22s

Good luck