Never seen this Graft done before. If you have been into grafting, it is quite normal to graft a small scion onto a large rootstock. As obtaining scion wood and rootstock of the same diameter is nigh on impossible.
But we were coming to the end of the grafting season and we only has relatively small diameter rootstock left over. We normally sell these are Grade B’s But we were given some rather unusual apple varieties. The only problem they were about 20mm diameter. So we performed the normal side graft, and taped them up.
Well we gave it ago, and we will see if it works. It does look really weird