found 2 years (66 and 67 I think it was) that do Prohibit slips
with flaps.

Was it actually in the "Operating Limitations" section, or just
mentioned somewhere in the text of the POH? I'm just wondering if the
pilot who wrote the procedures of the book exceeded his authority in
using the word "prohibited". I couldn't find a "prohibited" placard
on my CD of type certificates for any 172 model.