The hyphen is silent!
I have an 8-inch ... No, never mind that one now, but we do that in the States too, in some of the regions where they still use English from 250 years ago. In standard speech you would have a 50-fathom anchor line, not a 50-fathoms one, for instance, and some people still use the singular instead of the plural for some things.
I think it comes from an old form of English that has been preserved in some regions. Country folk in the States still call "braces" (American:"suspenders") "galluses", a useage still found in Scotland, I believe.
I noticed that just this morning, when Zulu Sierra something or other called level at three thousand five hundred "foot."
How about the Brits saying "short finals" instead of "short final"? Where does that one come from, given that most people only do one approach at a time? I know they are confused, but that confused?