help wanted to ID this plum rootstock, cuttings from root suckers

help wanted to ID this plum rootstock, cuttings from root suckers. It’s zone 3 hardy, few options right? Americana, sand cherry, mustang are the 3 I know of. Looks the most like americana, suckers like it.. but it roots super easily which seems different. Myrobalan isn’t hardy here to -40, but maybe it is? Suggestions welcome, plant from a garden center no idea what nursery, tree is 4 yrs old with no winter kill.