Gallienus Tet And A Couple Of Questions

I've been picking up a few Alexandrian tets recently, and the more I see of them the more I like them. I am happy with this Gallienus tet as it still has good detail, but the weight is under what appears to be normal for this issue. The RPC database gives weights of between 8.03 gr to 12.24 gr, with the average weight: 10.00 gr. The vendor described this coin as 6.9 gr, though my scales show the weight as 7.1 gr. Either way the coin is around a full gram less than what it should be.

Though irregular, the edge appears complete with no breakage and the coin appears to be genuine. Why would it be so light?

Gallienus Tetradrachm Alexandria 266-267 AD
Obverse: Laureate cuirassed bust right, seen from front. Obverse Inscription: ΑΥΤ Κ Π ΛΙΚ ΓΑΛΛΙΗΝΟC CΕΒ. Reverse: Nike advancing right, holding wreath in both hands and palm over shoulder. L ΙΔ (regnal year 14) in fields. Metal: Billon? Diameter: 21 mm. Weight: 6.9 gr/7.1gr.
Reference: RPC Online Vol: X No: - (unassigned; ID 75542); Sear #: 10581; Dattari 5254 (L IA); BMCG 2196; Köln 2938; Milne 4137

I read an article that mentioned some Alexandrian tets were made out of potin instead of billon. How can we tell which ones they would be?

Thanks,
Jim