I have two 645 with age unknown reserve batteries. Where did you get the correct battery?
I got it at Ace Electronics. I don't think you can get a CR1220 in Walgreens, CVS, or Wal Mart. They're not common. Try a specialty electronics store-the kind that sells parts for do it yourself projects. That's what Ace is, but I don't think they're nationwide.
Pentax 645, Pentax 6X7MLU, and many Nikons-F2 Photomic F2AS FM2N N2000 N6000 N6006 Nikomat FTN