Are you in the UK, HU? If so it might be a Green Gage cultivar. The fruit get a bit larger than they are now and usually ripen just when we go away on holiday, late July/ August ; - ) (Sods law) They are delectably sweet.

Yes plum thinning fruit next year can increase size

I don't bother to thin my greengage. The tree is loaded and it would take hours. They are not large fruit even thinned and I don't mind large numbers of smaller fruit. One fruit is a (largish) mouthful. Yum.

Thank you. Floral- yes I’m in the UK. Late July is when its ripe. I’ve been eating them but never knew what it was! I can now safely munch away.

Orchard fruit cultivars are identified using specific details of both the exterior and interior appearance of at least several examples of fully ripe fruits as well as the aroma, flavor, juiciness and texture of their flesh - in person examination is often needed.

A ripe greengage is unmistakeable. Which particular cultivar it is is a little trickier to pin down. But there are not very many to choose from and some are fairly uncommon. Probably no more than 4 or 5 likely candidates and some of those don't go yellow.

Don't eat the pits though. The original green gage is even higher in (poisonous) cyanogenic glycosides than cherry, but that's also where much of the flavor comes from. And some people consider green gage superior in flavor to all the other plums.

I am drowning in greengages this year. I'm having to give bags away and the freezer is filling up fast. Can't say it's ever occurred to me to eat the pits though. They're very fragile. They lose their bloom and start to ooze as soon as picked. These were on the tree an hour ago and already starting to turn themselves into compote.