Leningrad Black sweet cherry graft

lenningrad black sweet cherry on wild pin cherry. grafts finishing their third summer, still healthy, they flowered this spring. the sweet cherry leaves are much larger than a pin cherry. next to this graft are several other sweet cherries, swedish cultivars that have also survived. if two flower next year, assuming this holds up and were actually doing this, we're growing sweet cherries in an exposed area tho regularly gets a nights to -35C and always a few below -40C, well then I would suggest that it's either 1. because my grafts are five ft off the ground 2. these cultivars are cold hardy enough to survive -40C yet this ability is not well known 3. the grafts onto wild pin cherry are providing some extra cold hardiness 4. my temperature measurements have been off 5. some combination of cultivar, rootstock, local micro climate, height of graft are all playing together to make sweet cherries survive the past two winters where they shouldn't grow. so excited by this success, this summer I also grafted out some iput, big rupert, and one other I can't remember - all of which have some proven considerable cold hardiness, they say that Iput ranks among the best rating for this, though with sweet cherry it's not a usda cold hardiness zone it's a number scale up to 5 where 5 is presumably something like a zone 3/4