what does "5J" mean?

["Evangelos Makris" <xirias@hol.gr>]

There's something about Rostyles that I don't like. Even chromed ones like
mine. They look a lot like wheels from 70's Opel or Ford cars, bad. So I
may start looking for five ventilated disc type wheels from early BGT's.
Parts manual says "5Jx14". What does this designation mean and how can I
measure a wheel to make sure it's a 5J and not a 4J or something else?

Evangelos
73 BGT