Partly right, but that rootstock is Pond-Apple. I grafted that at Excalibur, on an EXPERIMENTAL batch of Pond-Apple seedlings.
If the graft is healthy a year from now, you should be good to go for heavy productivity, starting 3-5 years from now. Pond-apple trees are very different from each other, even varying in chromosome numbers. This seed source had not been evaluated, and most of the grafted plants looked terribly chlorotic, despite all my efforts with mineral supplements. So the plant you have is a survivor, apparently better than the majority of the batch.