The good ol' Focal Encyclopedia (fully revised edition, 1965) clears up the mystery.
A ferrotype, as all APUG users will know, is a direct positive on a dark-enamelled iron support, a sort of poor man's Daguerrotype in appearance, and often confused with Daguerreotypes by laymen. It is a variation of the Ambrotype and was introduced by AA Martin in 1852 or 53. This much I knew (except for recalling the exact date). It was very popular in the USA in the 1860s -- my wife has a ferrotype of one of her forebears, a Union soldier who died in Andersonville prison -- and survived for 'walkie' photography until perhaps the 1950s.
The very smooth surface of a ferrotype plate was at one time the most convenient surface for glazing, and the name was retained in the United States when polished chrome, stainless steel (and, according to the Encyclopedia) plastic were substituted for actual ferrotype plates. This was the bit I didn't know, and explains the use of a term I had always thought to be extremely misleading. Ferrotypes were of course generally known in the USA a tintypes, which is presumably why Americans don't see 'ferrotype plate' as confusing.
Glazed prints were much preferred for repro, which is why I used to run a Kodak 15-inch glazer, but with the advent of good RC glossy, there was no longer any need. Also, scanning is a lot quicker and easier than making a traditional screened neg.
Cheers,
Roger