I'm wondering if its a form of quince - Chaenomeles japonica. Has thorny branches, fruit is produced virtually stalkless, branches are quite dark in colour, drops its leaves in winter. Does grow in your USDA zone, although fruit production is sometimes compromised for various reasons to do with climate (fruit split, late cold killing the flowers before pollination). Tolerates shade very well. Fruits are edible, but usually used to make quince jelly, not tasting great raw.
Looks like Chaenomeles to me, but to make sure whether its an apple or not, cut the fruit in half, through its equator, so to speak. If the seeds are held in a five part case, its an apple. I reckon its flowering quince though.