I have the last three types you list BrendaP. The one at the bottom is exactly like mine, but mine hasn't turned as dark. I have polished some of mine up, and it looks like either type can turn rusty or darker sometimes. Some of the brand new ones have a bluish shine to the metal, I'm wondering if they are chrome-steel, that would make the stainless.
There are a lot of advice on plastic and metal bobbins for new and older machines. I have an Elna with a plastic bottomed bobbin case, it takes metal bobbins. The sides of the bobbins are polished and rounded off, no sharp edges like on 15 bobbins. They are very much like class 66, and there are rarely any problem with Elna bobbin cases, metal or plastic bobbins. Singer had similar models, with plastic cases taking metal class 66 bobbins.
Later there has been advice on using plastic bobbins in plastic cases, but personally I haven't seen much problem with it. Some have reported damage to their machine. I have a few Apollo bobbins for my 201, and nothing troublesome has turned up when using them. I have cracked a couple of plastic bobbins during winding, but the winder was out of adjustment at the time.