The Fujinon ES lenses were their basic lenses - 4 elements and reasonable performers. There isn't a lot of information about them online, but the information I've seen indicates that they use a 39mm thread, which is as close to a standard as there is.
Does it thread into a lens mount, or is it attached to a lens plate using a threaded retaining ring?
6x7 negatives optimally go with 90mm lenses, but many of the better 80mm lenses work as well. I have a later model 80mm f/4 Rodagon that easily covers 6x7, but I generally use my 90mm lens instead.
Congratulations on your darkroom acquisition - I can only hope that you enjoy it as much as I enjoy the enlargers I've had over the years.