That is a beautiful, smoothly healed graft, Sapote.
i am curious, too. Did you do a v-shaped graft in which you shaped the rootstock as a V, and the scion as a complimentary shaped wedge. Something like an inverted saddle-graft? Or was this a normal cleft graft - just a simple perpendicular cut through the rootstock with a wedged scion inserted? In the latter case, I always get two 'lobes' that look like small wounds, which were the top end of the split rootstock. At least the first year. They heal over quite quickly.