For class 66 bobbins there are at least three different types. The oldest I have that can be dated for sure is 1934 and I have exactly the same bobbins in my 1955 201. This type is put together by several parts of metal, very finely made, and the best ones I have. There is a tiny pin hole on each side for the bobbin winder.
The other types are made of one piece of solid metal, probably in a lathe machine or something like that. The older type ahs only the tiny pin hole for the bobbin winder on each side. The brand new ones I buy these days are very shiny and polished, made out of one piece of metal and have three extra holes on the side. The new plastic bobbins called Apollon are the same as these, all are sold as class 66 bobbins, they all have curvey sides.
Class 15 bobbins have distinct holes on the side, I think they are there to identify them easily. Elna metal bobbins have holes on one side to make it easier to wind and know which way it goes in the bobbin case. They have a groove inside the "hub" to click in place on the bobbin winder. The differences can have a function and not just for easy identification.