Beretta's website shows a pic of a SP I with the same checkering as your top pic so that seems to confirm what I've been thinking all along in that there is no definitive pattern for what checkering each model has. There might be..... but it isn't obvious and I think that you get what ever the stocker decides to put on that day!
I first discovered this phenomenon when I was rebuilding a SP that had a wrecked rear stock, I had to look for a while before I found a rear stock with the same checkering pattern as the fore stock.