I should know the root stock but I don't. The orchard is operated by a neighbor together with his larger operation and he bought full-size traditional Gravenstein for the 2012 replacements. He and his crew do the major phases - discing, mass planting, annual pruning, harvest, etc. I dig out stumps, water new trees etc.
Here is a link to UC Davis school of ag publications describing local apple growing:
Within that list, download the pdfs titled:
coststudies.ucdavis.edu/files/94appsprk.pdf
coststudies.ucdavis.edu/files/94appdry.pdf
These studies describe in detail 'best practices' that several of my neighbors described to the researchers.
I try to dig out every trace of the old stump and roots, and make sure the earth that was nearest the stump isn't near the planting spot when I'm done digging. This seems to avoid fungal infection of the new trees.