Okay, let’s see if I can shed some light on the issue. There’s a small choice, metal or plastic? class 15, L, or 66?
Metal or plastic bobbins are a bit of a muchness, personally I don’t think it makes a huge difference but try both for yourself and see what you think.
The class 15 bobbin (shown above) is probably the most commonly used. It fits drop in machines (where the bobbin literally drops into the top under the needle) and the side loading machine. By happen chance, we sell them online here
The class L bobbin is just slightly shorted in height than the class 15 bobbin.
The 66 class bobbin has a domed top and bottom but is of a similar size to the class 15 (almost but not exactly)
There’s a handy chart here showing a lot of modern machines and the bobbins they take.
Hope that helps!