My newest size die for the old Lyman 45 is something over 10 years old, and my oldest ones came from Herter's, so you know these things have been around for a while. And the last few years I have been seeing O - ring failures. I have tried getting O - rings from the hardware store but never quite hit it right and they have either been a scosh too small and leaky, or a scosh too big and get torn up on insertion.
I ordered a small selection of different sizes from "The O -ring Store" www.theoringstore.com/
I'm sure these were originally 1/16th inch rings so you'd think you'd want 1.6mm in a metric. But the 1.6 ers leaked a bit. The ones that seem perfect are marked as "1.78mm X 14mm" from their Buna-N 70 Duro selection. No more scavenging O - rings -- I can replace them as they fail. I'm a happy li'l camper!