To my eyes, The fruits and leaves looks more like a Prunus Sp, like choke-cherry (Prunus virginiana), bird cherry (Prunus padus) or other similar species.
Huckleberries has a ring/crown thingy (calyx scar) at the top of the fruits, like other Vaccinium (Blue berries) species, and the leaves has very small teeth at the margin, almost looks like smooth from a distance.
The mystery fruits has bit obvious serrated margin

To better ID plants, it is better to have more images, like leaves and fruits still attached to the stem, branch, bark, whole plant image etc. Detached leaves and fruits can be ID sometimes when it is obvious and unique, but in many case, it is not enough.

Leaves look more Purnus-like to me, too. Every berry is turned over, and we can only see the bottoms. Can you turn some so we can see the tops?

Also cut one open so we can see the seed(s). Blueberries seeds are tiny (hardly detectible in the mouth if you eat a berry) and relatively soft. Usually 10-40 seeds in a berry. Huckleberry seeds are much larger, hard and stone-like. Seeds 10 or less in a berry. Purnus seeds always are one to a berry.

Hi! Here are more pictures!






(two tops two bottoms)

Looks definitely to be a Prunus sp. Note the arrangement of the fruit on a terminal spike, following the flowers.

What's native in Montana? Possibly Common Chokecherry (Prunus virginiana)...