Water pump pulley self destructed-HELP!

[dr.delco]

Hi everyone,
I was wondering if anyone has experienced this or had it happen to them.
We're talking about a 64 truck with a stock 230 inline 6. Today my water pump pulley decided to separate itself from the water pump shaft. The water pump shaft spins easily, I do not believe it locked up. I have had many old cars and trucks and have never seen this happen. Any ideas as to the cause? Metal fatigue on a 50yr old pulley? Also, does anyone know what would be a good replacement for this pulley. Inline 230's are not that plentiful in the boneyards. It measures 7 1/8 in diameter. Thanks