Hey I'm about to post my first reply answering someone elses question (well sort of). I'm not asking a question or thanking someone for answering my question. This must be a significant occassion!
Anyway, I too have noticed that Airstream hitch ball heights peaked in 1967
, at least according to the info on the Airstream web site. Not sure if all this data is valid, but I can confirm that my '66 Owners Manual and trailer seem to agree with the '66 hitch ball height posted on the web site (20.5").
I'm not sure what the reason is for the changing heights in the late 1960's, but the switch to the Duratorque axles isn't it, because the changeover occurred well before 1967
. I have heard the switch was about 1961