A Suspicious Ptolemaic "Tet"

Hi all, I've been asked to help out a local dealer ID a batch of ancients they've just bought in. Most of them I've got a pretty good idea on, but this one worries me. But I really don't have enough experience handling late Ptolemaic pieces to know whether or not this one might be "within tolerance".

It looks like a base-silver tetradrachm of Ptolemy XIII, Year 2 (which would be circa 50 BC), or maybe Ptolemy XII, but it's much lighter than any I've come across in my references: only 9.41 grams according to my balance. It doesn't appear to have lost enough metal by corrosion, damage or wear/clipping to account for that much weight loss. There's also a very odd "rust" spot on the edge which worries me, though if it really is debased silver it could just be the "red death" Egyptian debased silver alloys are prone to.

My magnetic silver tester says there's probably a tiny bit of silver in it, though nowhere near as much as the Alexandrian tet I used to calibrate it with.

Opinions? I'm guessing "fake" at this stage, but I'd hate to condemn a perfectly good though heavily debased "Cleopatra coin" based purely on a suspicion.