Viewfinder info requested

[GAPiccagli@xxxxxxx]

Hello list,

I notice that Linhof Universal Viewfinders crop (no pun intended) up 
repeatedly as favorite solutions to viewfinders for homemade and pano 
cameras. I am confused about compatibility among various models and their 
masks. Can anyone steer me to a place/site/source/book/publication that 
details the different models and their compatibility with various masks? Or 
is there a simple, short story somebody can forward?

In particular, there seem to be older and newer versions, 2X3 vs. 4X5 
versions, etc.

Thanks.

Giorgio Pic
Foggily yours in San Fran