That's interesting, Richard, as I hadn't heard of them. We grow Victoria plums.

I looked at Google images, and sure enough, there are some depicted just as you describe, but also some that are not unlike Victoria plums, and, reading some of the articles, the suggestion is that they are early plums, so that may be why they are a little sharp.

It is very strange that if both colours are indeed 'Opal' plums that they can vary so much.

They will probably make some good plum jam - I'll have to look out for them!