Sorry Angus, even if you asked about the best vintage of a nice italian wine, i'd be at loss as well
When i write that the age of a lens counts (and sometimes could even make a great deal of difference), i mean decades, not years!
Some lenses were made during a long span of time, the name was the same, and no variations of the manufacturing was disclosed, but it's known that the cement was changed, and the coating process improved.
Later on, some types of optical glass were discontinued, so the companies had to quietly replace them with new versions, and of course the elements had to be recalculated.
I have very little knowledge about USA-made lenses, and the very few notions i have are easy to find with a simple search.
Nevertheless, you can't go wrong choosing a later Ektar vs an older one.
The CAMEROSITY code will let you know the year of production.
If you really want a multicoated 90mm, which is light and small, and of course second-hand and conveniently priced, definitely you better be patient.
Other tha a late Congo/Osaka/Prinz, there could be a Topcor/Horseman (MC?), and there are the Geronar/Caltar wides (90mm f/6.8 IIRC), but not sure if they are MC.
Better check with Google.
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