Well, it's settled and proven knowledge there were at least 5 versions of the 50/1.4, differences being number of elements/groups (7/5 vs 7/6 I believe, or something along those lines). There were differences in coatings, as well (at least 3 different versions there - single coated, MC (marked MC) and later MC, called "new MC" by Olympus). This is all well documented and proven, via multiple sources, not the least of which are the actual parts diagrams and service info from Olympus themselves. Mechanically they are all about the same, with a nod going to later ones for more well-sealed groups.
In practice, there will be differences optically, but they may be too subtle for some users to notice. I've had multiple versions, and given the reasonably low difference in cost, it makes sense to go for the best (>1.075m serial #). The difference between the latest ones and earlier ones is usually < $100 so no sense fooling around, go for the latest and greatest version.
-Ed