I decided recently to replace my Zuiko 50mm f/1.4 single coated lens with a newer Zuiko 50mm f/1.4 multicoated lens. I wasn’t able to find one with a serial number over 1.1m as those are supposed to be the best, though mine does have a serial number of 1,05x,xxx. I am going to try to compare them over the next few days before I make a decision about selling off the old lens (the likely result unless somehow the old lens proves to be better). I am going to do some testing comparing the two lenses at f/1.4, f/2, f/4 and f/8 using a tripod, mirror lockup, cable release and some sort of test target.
I haven’t been able to find out any information about if the lenses actually had any changes to their optics over the years other then a change in coatings, but I would imagine there was probably some sort of change. Cosmetically the newer multicoated lenses are 4mm longer, though most of it seems to be in the filter mount. The weight is identical. The coatings are actually pretty similar in apperance, though the multicoated lens has a bit of a greenish hue that the single coated lens lacks (hard to tell from the pictures).