I discovered that I had saved some GoldRush apple scion in the refrigerator. They were in a ziplock, fallen at the back of the dairy drawer. There was one remaining tier, the 3rd up, to replace on the Redlove Calypso espalier, so I made some grafts.
Where I could find s stem to graft onto, I used cleft grafts. This is a century - old illustration. It hasn't changed.
The main thing with this carpentry, is at least one side requires the cambium layers of scion and stock are pressing against each other. Even better if for both sides, but not required.
There were some locations I thought might be better, but no branches. For those, I wanted to do a traditional side graft. An illustration from the 1860s.
I've done this before, but with bud grafting. That was cherries, in June. This time it's apple in April. I had difficulty getting the scion to fit in the slit, even though I was able to expose the cambium layer, I tore some of the thin bark and cambium and did not wind up with success. So I treated that area of the branch as a self-graft, placed the layers back together, and wrapped snugly.
Then I went with my earlier idea, and cut a thin tongue in the bark, again exposing the cambium layer. Insert the scion into that slit, Wrap snugly, as best I can.
Will they tKe? Watch and wait.