This 15mm plastic safety eye shows before and after the washer is properly installed.
There is a difference in washers. The plastic washers are for the "ribbed" stems. The metal washers are for the "smooth" stems.
The stems can be small on the different sizes, 12mm + have the washers with notches as in the picture above. 6mm & 8mm have the small "dome" washers and the 9mm & 10mm will have the larger "dome" washer. Regardless of the stem or washer, the flat part of the washer always will go on first so the flat (back) of the eyes will match to the flat part of the washer.
They are suppose to be hard to get on - and impossible to get off. If you can get them off, there is a problem, usually the washer is on backwards, or you are using the wrong size washer.
Yes, different manufacturers use different shape washers, there is no standardization.
