Well for rootstock you usually have to stay in the same species - so not plum . I assume you see thorns . So on pear it could be flowering pear (Callery) or Harbin . One last year is on what I suspect is a wild Callery x Harbin mix . This years graft is on Harbin = good to zone 2 . Apples were often on seedling apple . However other clonal stocks were in use . Antonovka is a Russian seed grown cold hardy standard size apple stock still in use .Thanks. The guy I got them from was asking. If you get down this way this summer, I would like to show you his old trees. I think they are starting to get runners off the root stock.
What would they have used for root stock 30-50 years ago? It almost looks like a plum of some kind.