Just to add to the confusion, there was a time, early on, when it wasn't "Solar Engines of Phoenix Arizona", but rather "Thermal Energy Engines of Phoenix Arizona" as I have a Solar #1 with that cast into the underside of the base that is numbered in the 5xxx range. I think it more than likely that the Morgans, or possibly the Griffins, actually bought the molds and tooling along with the manufacturing rights from Davies Charlton which was at that time a bit past their prime and starting into a long decline.
I think you are right about the 1977 ad being a "trial balloon" before they had all their ducks in a row and their final product actually in hand. It would be exceedingly interesting if a SEoPA or TEEoPA marked engine were to show up, that was actually configured just like the D-C engines!
I'd still like to see a copy of the Finkelstein book as well, and see if the Ross book is just a rebranded version, or wholly different?!?!