I don't see the transition as odd at all. In my mind, there was no transition, actually. As I see it, all the belts (beltplate style with the buckle on the right and open face style with the buckle on the left) are all "right handed" configurations.

With the beltplate style, you hold the buckle in your right hand, pull it across your body and hook it on the tab end held in your left hand. The right hand does the finer muscle skill task of getting the tab into the slot whereas your left hand just holds the slot end still. In the open face design, your left hand again holds the left side in place...in this case an open buckle loop with one or two prongs... while the right hand feeds the leather end through, pulls it back to the right to cinch it tight, and then feeds the end back under the other half of the buckle or through the leather keeper...or whatever keeps the belt end from flopping around. Again...all the finer motor skills are accomplished with the right hand. It's really quite simple and logical in my mind.

If you look at military beltplate style belts, you will find almost all of them (I'm sure somebody will find an exception...which is why I threw the "almost" part in there) have the buckle on the right and the tab on the left. US swordbelts, British swordbelts, etc.