Mike, I'll try to answer all of your questions:
1.) No, there are only black WBs made from iron and brass (rare!), so your badge is a "real" silver Wound Badge
2.) Yes, there are hollow made silver WBs.
3.) Non-magnetic could be silver plated brass, or... silver! To test the last you could ask a jeweller for a test or you could make a small scratch on the reverse to see if there is another metal under the silver, but this method may be a little rough
Not all real silver examples were stamped!
The first thing I would do is to grab a good lens and check this badge for any parts, were maybe a (possible) silver-plating is polished off by wearing that badge. So I identified a "real silver" WB from Jörg Kalies as a silver-plated brass badge some weeks ago. The plating was removed at the air-hole of the M16 helmet and there was brass visible!
I hope this will help you.