Concave grooves.
When you take the time to carefully study these pieces you will quickly notice repeating concave groves. There are several different types that I see repeating. One type has a sharp edge and was most likely used as a spoke shave. A second type is wider, smooth and polished and was possibly used to soften hides. These concave grooves come in a fairly limited variety of sizes. The majority of all the pieces I find have at least two repeating sizes of these grooves and the radii of these concave grooves are copied quite closely from piece to piece.
Although hard to see due to the different angles all four of the pieces below feature the same size of concave groove. This size of concave groove is the most common I see. It’s amazingly close to the same circumference as the pad at the tip of my thumb.
I have over 40 years of experience working with hand tools and exotic wood and I can confidently say that if you were to make 4 different staves using each of these as spoke shaves you would not be able to tell them apart or which tool made which stave.
It is truly amazing how close some of these features are copied from piece to piece.
Here is an authenticated example I found online. Notice the similarly orange coloured residual material.
