If these 1" survied dropping like other, then your assumption is correct that these have the potential to ripen. The next survival is against the birds and squirrels. Good luck.
My plum trees, cherries and apricots did no show a single bloom this year (oh yes, three small flowers in mid June on Northstar.. bizzar!). It appears something unusual has happened in the Northeast this year(like bad laste frost).

Lipother:

Yes, you can absolutely assume that only the 1 inch fruits have a chance to become ripe plums. The others were either incompletely pollinated, or the tree was not ready to hold them, or both. Lucky for you. If a plum tree holds all the fruit it sets, there is major and repeated thinning to be done.

Plums are never out of the woods until they get by the insects, and, as the Ottawan observes, the birds and the squirrels. Good luck.

Don Yellman, Great Falls, VA

You might want to add wind to the list of perils. I found several of the larger plums on the ground today. :(

Or hail

Some of my plums look like this and yes, the larger one's look like will stay, but this still looks weird without leafs.

{{gwi:116583}}

Konrad