I think they're Northstar Cherries! Their mature height is 6-10 feet. They are pie cherries (ie sour), and cold hardy. Yay! I've got 6 of them.
I found the final clue in my new favorite book: Back to Basics, A Complete Guide to Traditional Skills, 3rd edition, edited by Abigail R. Gehring.
Here's a photo and blurb from www.fast-growing-trees.com/
Famous for its unmatched beauty, but demanded for its delicious, juicy cherries!
Introduced by the University of Minnesota in 1950, this dwarf cherry tree boasts attractive, richly colored bark and petite white flowers in spring, followed by an abundant production of juicy, dark red fruit.
In fact, those who enjoy the many ornamental qualities of the Crabapple tree will love the North Star, as it has many similar physical characteristics.
These plump, tart cherries are considered ideal for baking, but are delicious eaten fresh off of the tree as well. Fruit ripens in late June to late July.
Reaching a mature height of 6-10 feet tall, this cherry tree is ideal for small spaces and works well into even the most established of landscapes or tiniest of yards.
Extremely hardy and self pollinating, the North Star could not be easier to grow! This is one of the few cherry trees that actually don't need a pollinator... so one tree will give you all the cherries you can handle!
Both cold hardy and disease resistant... very low maintenance!