If you can find one, a Pro-SD back will be newer, and give you the most functionality.
I note that KEH has a Pro-SD back listed currently. It is somewhat expensive, but they often don't last long on that site.
A Pro-S back would certainly be functional.
Did your camera come with the rotating back adapter? If it came from KEH, the listing would have usually said so.
Well, i have to check out my camera again, i bought mine from KEH in fact, if they didn't list it as LN then i may think it is NEW, so i will try if i can find the back from same site before it is gone, i found the back on ebay new and used, so i will keep my eyes on all sites i can find.