66-68 are the years with the thin frameless curved glass that uses a somewhat-complex latch mechanism. The '69s used the same style as the '70s trailers, with a metal edge all the way around the glass.
Not that it's exactly what you're asking about, but I think the shape of the "wing windows" at the front of '69s is slightly different than the '70+ windows, but this is a part that doesn't open, so it's only an issue if you have to replace one.
Il Carriaggio — 1975 Argosy 24 | Il Progetto — 1976 Argosy 28 Center Bath | WBCCI# 15566
He has all of the virtues I dislike and none of the vices I admire. — Sir Winston Churchill