Not one response? I guess I will look for a better forum elsewhere.

Well the fact is that your question is pretty much impossible to answer, but I wish you well.

Is your picture current? it looks like you have mulch or dirt still piled too high around the trunk - the root flare should be visible.

Did you buy your Santa Rosa from a nursery? I would contact them for advice; or your county extension. I agree it is not a Santa Rosa (AFAIK); but I am not that well versed in plums to suggest what it might be. Googling a description of yours might come up with some possiblilites. IIRC I had a plumcot many years ago that looked similar.

It could be that your Satsuma doesn't pollinate your mystery plum, but there must be another Japanese plum in the vicinity that is pollinating the Satsuma. I have one Toka plum that produces one or two fruits (I've had it only 3 years) but I know that my other plums are either European or Native, so they probably don't pollinate it.

Thanks for the informative reply. I didn't realize the issue with the root flare, I'll take care of that. We have a plumcot nearby, that yields plenty of fruit (also needs its flare uncovered). I am not aware of any plums in nearby yards that might be pollinating the Satsuma, so I assume it's the mystery plum that's doing it. BTW, the mystery tree flowers three or four weeks ahead of the satsuma.

We've googled the heck out of it looking for similar fruit on the outside and inside but so far no luck. The place we bought the tree from is no longer in business unfortunately.

I removed the red bricks, Does this qualify as a "visible" flare?

Yes, I think so.

I wonder if your plumcot is doing the pollinating? Is the mystery plum still in bloom by the time the satsuma is ready for pollination?

If it blooms weeks before the satsuma (which maybe indicates that it is an apricot/plum hybrid, because my apricot always blooms weeks before the plums) it could be that the satsuma is just too late to pollinate it.

Another possibility is not enough hours of sun on the mystery plum, or incorrect pruning - but since your other trees are producing, I doubt that pruning is the problem.

Perhaps if you sent your photos to one of the big nurseries (Stark Brothers, maybe?) or one of the university extension services that deal with fruit trees a lot (Oregon or Michigan come to mind) they could at least narrow it down.

As I recall, the mystery plum blooms first, a week or so later the Plumcot, then finally a further three weeks later (so about 4 weeks after mystery plum) the Satsuma. The plumcot has never managed to pollinate the MP, but something sure is pollinating the satsuma. I assume the MP?

All of our trees see a lot of sun, here in No Cal. MP is not shaded, it gets probably 10 hours per day of sun. It grows like a weed, there's a lot of fresh growth on it now.

I would be surprised if trees that are blooming a month apart cross pollinate, but you are in a very different climate than I which will have an effect. I suppose it could be that, based on what I've read, that the MP still has viable pollen for the Satsuma, but its ovules have aged out of being able to be pollinated by the Satsuma. Those sources (university extensions) say that pollination usually needs to occur within a few days of the flowers being fully open.

All speculation on my part, I am only a backyard gardener playing around with fruit trees.

More evidence perhaps that the MP is not a Santa Rosa, as a satsuma is suggested as a pollinator for a Santa Rosa?

It doesn't look like any of those on this list

Plum Tree Varieties: 23 Different Types of Plum Trees For Your Garden (allaboutgardening.com)

Closest is elephant Heart and Friar but both of those apparently cross pollinate with other Japanese plums - like Satsuma.

I'll keep looking.

I would also think that if MP is a plumcot, pluot or aprium, your plumcot would pollinate it. Why it blooms so early is a mystery!

I saw one picture of fruit from a "July Santa Rosa" which looked promising, but others that did not. There was very little online about the "July" variety, but at least the name implies an earlier cycle than our Satsuma which is not quite ready.