Thanks for following this thread up everybody.
It does sound as though you are narrowing in on the locallity and the date range .
Have you seen Eli Whitneys barn at New Haven? built in about 1815.
Whilst this will not be the first square rule frame it would give a benchmark on how well developed the method had become by this date.
I just built a small square rule frame the other week, using a lot of similiar tools and techniques to scribe rule until you get to the housings, where as you know the two techniques then diverge. Interestingly it does not appear to be much quicker for the primary timbers. The real increase in speed is for the smaller secondary timbers. We were not cutting housings on the layout faces, and just snapping one (or at most) two chalklines on the timbers as the layout required. No trial fits, and luckily no mistakes! Shoulders at the joints were not as good as with scribe rule.