Thank you for the quick reply. Below are the answers to your questions and I also attached additional photos.
There is a notch on the back side at 0 degrees and a notch on the front at 180 degrees. To the left of the front notch is the number 3.
I have searched all over in all kinds of lighting and see no other numbers. That is what baffles me from what I have read and you have provided.
The outside lip is very thin.
I am told the person who gave it to my grandfather had a Willy's Knight but no idea on year or validity.
Outside Diameter: You said 10-1/4" (It very will could be 10-9/32, I didn't measure in 32nds)
Prism Diameter: ? 9-15/16 as close as I can tell
Notch location: 0 to 360 degrees, 0 is at 12 o'clock.: ? Rear at 0 degree. front at 180 degrees
Part # on outside edge of lens : ? Sorry, nothing but 3
Some "Guide Lens" have a mfgr # like "GLnnn" (n=digit) Again nothing.