I've spent as much time in apple orchards this year as I have hop fields. After enough time, you start to notice the little things and ask the right questions.
What you see here is a splicing and grafting technique whereby the rootstock of one apple tree is used as the base for growing an entirely different varietal. The technique is distinguishable by the yellow seams, which is actually a type of "glue" that prevents infections in the otherwise open wounds of the frankenstein-like procedure. This one is quite an old scar.
Grafting helps take advantage of the already healthy stock in the ground and enables a new trees to bear fruit within a year or two without replanting. It's an old-world technique, and keeps the genetics diverse and robust for generations.
This particular graft is from E.Z. Orchard in Salem, Oregon, a farm still in the family since the 1800s.