A 50 Hz motor run on 60 Hz will have less iron losses (run cooler) Generally a bad idea however to run a 60 Hz motor at 50 Hz.
..and depending on motor age and how "International" the firm that selected any given motor eg; "bought it in" did not themselves make, was?
It MAY have actually been optimized in the Iron and Copper for
55 Hz to begin with. Slightly imperfect as to match, either way, but ONLY "slightly" and also equally comfortable on either supply. All that BEFORE
VFD - as in "Variable" were yet a major factor.
Splitting the difference was not all that uncommon as makers try to keep costs of differentiation down and go for twice the volume, same-same parts, not have to make and stock and assemble minutely different ones.
Or - face it - just jigger what they stamped on the dataplates within their reserves. As they were ACTUALLY more likely to do instead, anyway!
You'd have to know bean counters, "ULM", and their effect on Product Engineering?
North America with US // Canada, and Japan (who use both, internally), being large markets, before even crossing the Atlantic? There's a fair chance Hardinge as a firm is one who may have done the 55 Hz schtick. Whether they even KNEW it or not.
Not that there is much to worry about, even if not.