That’s a good question Steve.
I’ve noticed up on eBay and other places that people describe them both ways. Confusing. In your case they are most likely listing the barrel ID size in MM. which may be too small for your 18 size Elgin. You’ll see them as described in watch sizes too…ie: 16s 18s etc.
I think reasonably priceed good PW spring winders are getting hard to find. There are some new ones by Bergeon but may have to mortgage the house.
If anybody knows of a source for them PLEASE let us know.