I got the info from a very interesting 6 page thread on these badges on GCA where i'm a member. I don't know if the link works without joining (which you can do for free) but it's a fascinating read with various badges, including Stalingrad field made badges (purportedly) and others including a marked '30' Vienna Mint one.
GCA Forum | Access denied.
Some of the postings are in German, but that's not too much of a problem really!
EDIT: Looks like you may have to possibly be a paid up member to view it, i'm not sure.....Buggrit.