I think I figured it out a few years ago (Mount Royal), though you didn’t seem convinced:
At least they were good about sending me another tree, hopefully the correct one now. And the original produced some very good fruit. Just not what it was supposed to be.
Ouch…Did you send them pics?
They say they do, though it wouldn’t surprise me if they source some from other places too, as this link says they dug 6000 trees last fall, which seems a bit low for a nursery. They used to ship in the fall, but I don’t think they offer it anymore, as it was a bit rushed for them. https://www.grandpasorchard.com/Blog-Post/How-Grandpa-Harvests-and-Stores-Fruit-Trees
That’s because I managed MR at the Heffernan orchard for about 20 years and this plum is not much like it. The false Valor bears 3 weeks sooner and is a much shier bearer of sweeter plums- they are exceptionally good. I have plenty of experience managing this one as well by now and in spite of its inconsistent cropping have grafted it onto an Empress in my own orchard.
One property I planted it I named it Julia in honor of the far better half of the owners. .