I recently picked up a lot of old gold-filled items. Included was bunch of parts of old eyeglasses, the middle parts that have the nose rests. A few are marked as 1/10 12k gf, but most aren't marked as to content. I found one full 'pince nez' frame that is marked 10k, so I decided to test the unmarked pieces. Every piece I test reads as 10k gold on my Mizar ET18 tester. (i know). I am filing so deep that I almost cut them in half, still says 10k. So I tried one of the marked 1/10 12k gf pieces, filed deep into it, it says it's 10k too. When I tried some other karat gold and gold filled, the tester read correctly. It's just these eyeglass parts. Any ideas??