I was doing the perpetual seach for 7.5 Swiss GP11 this past Wednesday and happened to come across newly made K31 Mags, they are blued, hold the standard 6 rounds, have no serial number on the bottom and are a fine quality steel mag. I ordered two and they arrived yesterday(2 days early!?!), with 100% fit and function in my 1948 K31, I'm a happy camper.
The only differences I can tell between these new mags and my original serial numbered mag is a very slight weight difference(new mag is a tiny bit lighter in the hand) and a dimple on the follower of my original mag is merely indented circle on the new mags.
The retailer is called Keepshooting.com, and the mags are listed as "Schmidt-Rubin K31 Magazines" at $44.95 per mag, shipping was $5 and change to my locale total. I have no affilation to this business, just happy with this product and thought perhaps other K31 Swiss owners might be interested.
Best Regards,
Hking