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.
Zero Gravitas — 2017 Flying Cloud 26U | WBCCI# 15566
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