Thanks so much for this - I have been away and have not visited the forum in the past week.
Last year I had the opportunity to acquire two historic pianos from the Finchcocks musical museum in Kent.
One is an 1859 Broadwood concert grand - the one hired to Sir Charles Hallé for the Hallé Orchestra and which was purchased by a Manchester businessman for 250gns as it was the most amazing instrument he had ever heard, and the other, a grand of 1802 by Stodart. This can be head on https://www.youtube.com/watch?v=gJonwRwgaeo
For the past decade I have been using the Kellner "Bach" temperament, a temperament based upon 7 perfect fifths and a variant of Werckmeister III
Both these pre-1870s instruments have something significant in common - the Quint harmonic is the principal and prominent harmonic and the tierce is completely absent. In subsequent pianos of the 1880s the Tierce comes forward in the sound often more strongly than the Quint.
The prominence of the Quint in both these and other earlier instruments is suggestive of its importance, and getting as many perfect fifths in the scale tuned spot onto the harmonics of the strings both increases resonance and provides key colour as the keys open doors to different sounds on the vibration spectrum, key colour in "chromatic" music, "chromatic" being not semitonal but related to the Greek word for "colour".
The nomenclature as well as the evidence of Haydn, Mozart, Beethoven, Schubert being particular about which keys they used for what leads to a certainty that such tuning is what they expected.
It's for these reasons that I'm now starting to talk not about Tnequal Temperament but "Classical Tuning" as these instruments and the change of instrument construction with the adoption of Equal Temperament make the nomenclature of "Classical Temperament" a certainty.