I may be wrong, but based on my understanding:
A) UPO = latest version of the 45mm Planet Ocean rep. It is labelled ULTIMATE over previous versions of the 45mm PO because it has the correct Omega logo, better AR, correct HEV, more accurate bezel ring and pearl. Any previous versions of the 45mm PO may not have these all issues corrected. All depends on where its coming from.
Note: The Ploprof has a similar label where the ULTIMATE aka V2 Ploprof has the better crystal, functional bezel lock and etc over previous versions.
B) The crowns stripping seem to be a common issue on a lot of PO's. I think its a result of the quality of the tube construction...the tread design is different than the typical crowns like you find on other screw down crowns.
The ULTIMATE reps are just more (and the latest) accurate reps over previous versions.
Note: The 42mm PO's never went by the label ULTIMATE. The latest and greatest version of the 42mm PO rep would be the 5TH GEN version...again same logic as the UPO. Just a different system for labelling the different versions.