Oregon used to have a pdf catalog sorted by OEM or blade dimensions and mounting hole diameter.
Measure yours and see what you can find. FYI, the length is corner to corner.
Don't know what to make of 20.8", which is between 20-3/4" and 20-13/16". The width and thickness are also important.
I just checked, and I don't know about a standalone pdf, but they do list blades by dimension on their www site. Good luck.