Had the same problem recently. While searching I noticed that a couple of sites were calling them M8 form G as well as penny washers. It turns out that a standard has been set that defines the outside diameter and thickness. More information on Wikipedia.
https://en.wikipedia.org/wiki/Washer_(hardware)
Scroll down to type and form.
The standard applies to all makers so one company's Form C washer is the same as everyone else's. Tables for the different forms are readily available on the internet.
You learn something new every day.
Al