You’re right, there isn’t a dedicated thread to the Optigun/USB2Gun method.
There are a lot of separate posts about it that could be cherry picked through to gather most of the information required.
Compared to other interface methods, this is quite obscure though…. Probably due to only being usable on CRT’s and it’s availability.
I sold my Lightgun arcade machine using one of these last year. I did have 2 of these at one point, both gave out different results and had to be calibrated differently from what I remember. Either I had a slightly duff one or these boards act differently from one board to another.