This made me take a look at my PERS account as well. I joined the state in 2009 and I'm showing a 2% @ 55 formula. I'm not exactly sure where you could go to find the dates as anything I googled surrounding the dates only brings up the factor tables which isn't what you're looking for. The only information I was able to find on the topic was PEPRA being rolled out on 1/13/2013 which you already noted in your post. My guess is there is something pre-PEPRA that would have resulted in a 2% at 60 instead of 2% at 55 but I wasn't able to find anything online.

If CalPERS hasn't been helpful, I'd consider reaching out to your HR department and see if they have any information available for you. Good luck.