The rootstock I have is M7, should make trees at about 70% the size of those grafted on "wild" rootstock.
My source also had MM106 which should be about the same but better for different soil (can't remember which) and M9 which is 40% of the wild rootstock and definitely needs staking.

I think the M series is named for the East Malling horticultural research center where they're been developed, but I could easily be wrong.

I wrote the previous before following the link, funny to see I got something right, including picking the right rootstock for my soil.

Due to space constrains I have to keep the trees small, but picked the semi-dwarfing rather than the fully dwarfing rootstock because I'll be training them in espaliers, so I think I'll need a bit more vigor to play against than the dwarfing rootstock would give.

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