Hi 20 -- Good to hear from you!
I have never had an A-410 to measure for myself, but if I were a betting man, I'd bet the screens were tapped at 23-25% of the winding in that transformer.
Dave Hafler was the real motivation behind the development of UL. While Keroes was certainly involved, his motivation lie primarily in developing TV horizontal output transformers! So it was Hafler that ultimately developed so many of the popular UL operating conditions that came to be known with various output tubes.
Ed teamed up with Dave early on in 1955 to form what became Dynaco. His contribution was the development of the pentode-driving-a-triode-phase-splitter-all-in-one-tube circuit, which landed him the position of chief designer at Dynaco. Combining his input circuitry and Dave's output circuitry was a natural, and resulted in the original Dynaco MK II. As they say, the rest is history. How many times has that basic circuit arrangement been copied!!
In any event, by 1961, optimum operating points for UL were well known, with Hafler's name and UL basically being one and the same at that point. I have an original copy of the article you refer to, and since he did not represent himself as being part of Dynaco in it, it is unclear what his intentions were. He was walking a fine line though because while he mentions "Tests showed no advantage" (in using the A-410's UL taps), there are in fact clear advantages to using a tapped screen connection that he does not mention -- primarily that being the reduced output impedance and improved damping that connection provides. This is very significant when driving a loudspeaker load.
We'll never know why Ed "deviated" from the Dynaco formula in that article, but it could be anything from trying to show that 6BQ5s can produce good performance in pentode mode (which they can), to simply trying to establish his own name out from under the Dynaco spotlight. I doubt it was an effort to troll for interest in such a design, as by that point, 6BQ5s, UL, and Dynaco products were well established. It may be he was tipping his hat to his boss, since at that point, there were virtually no UL 6BQ5 designs in production (in retrospect, the Dynaco and Heath versions represented the lion's share of such designs), and the Dynaco UL 6BQ5 products were just getting set for release.
In any event, 6BQ5s and 7189s are one and the same when it comes to operating characteristics (not limitations) so that likely had little to do with the matter. As for the transformer though, it would be great if someone who had an A-410 could measure and report the tap placement on it, so the question could ultimately be answered!
Dave