Duane, thank you for another inspiring post! Since I saw the end plates for the first time I'm wondering what Ed intended. I had the same idea as WEDJ and I like to comment even though I'm not an aero engineer (industrial instead).

First, I wondered why Ed used a thick horizontal tail but the old slab vertical tail. The only explanation I have is that it's a by-product of the thick wing. There's a simplified rule that the tail should not stall before the wing. A thick airfoil makes for big maximum lift at high angle-of-attack, though. Usually the horizontal tail has a rather small aspect ratio to accomplish a stall at higher aoa than the wing. Now the carrier wing is so thick that a thick tail airfoil may be required in addition.

Following this simplified "theory", one could assume that Ed still felt that the horizontal tail stalled or was not effective enough. So he replaced the small stab tips by end plates, effectively getting more tail effect at even higher aoa. Maybe he fitted them toe-out (wider at the front) to avoid the nasty flow separations that would otherwise develop in the corners between stab and end plates. I don't think a small toe-in would have a noticeable effect on directional stability, either.

That's the only "theory" I had so far, and it has a major drawback since Ed fitted the end plates only after flying the T2 with the new thinner wing for a long time. So the reason for the end plates could be directional stability as well. After all such things are used on floatplanes for that reason, and maybe Ed was satisfied with the thick horizontal stab but not with the slab vertical.

As an afterthought, he might have complemented the small and ineffective vertical tail by the side force generators, as they are now called, or simply fins, like those used on many floatplanes to augment vertical area and directional stability. The small horizontal tail tips were not really needed (and the end plates gave more horizontal effect), anyway. And it would have been much harder to replace the whole vertical tail by a new one since it's tightly integrated with the fuselage. Ed did replace the slab tail by a big and thick-airfoiled tail (later?) on the unfinished fuselage.

It's all theory, I know, but seems consistent and is more than nothing. Any more bids?