Hi @Balance,
I've been a long time lurker on this site, but finally bit the bullet and made an account so I could respond to you. I agree that the skin on the trifoliate fruit in my pictures looks a bit rough and thick compared to what's typical, but ironically it actually came from the exact tree that is the subject of this post. I saw this tree at the arboretum one day and was with a friend who had sampled it before. He told me this tree produces seedless fruit (just like you experienced), but I went ahead and collected a couple anyway. As luck would have it the fruit actually DID have seeds (that time at least) and so I was able to start a bunch of trees from it. I did not detect the off flavors people generally associate with this species, and I looked at the accession card for it to see its history. This tree was one of three trifoliate seedlings donated to the arboretum by Lester Brandt of Puyallup WA (originally from Tennessee) in 1947. I suspect that it is not a hybrid, but rather a different genetic lineage than what is more commonly available for this species.