Which center filter do I need?

[Massive Si]

I have a cambo 35mm digitar for my IQ350 and wondered if I should get a center filter for it (if I can find one)

I am able to balance out the vignetting somewhat with LCC in C1 but am thinking if I should try and reduce the stress on the blacks, especially when shifting

so if I need one and can find one, which center filter should I be looking for?
The thread is 52mm, and are the center filters themselves threaded? so that I can mount my Lee filter system?

thanks

 

[Paul2660]

If this is a Schneider SK35, I would look for a CF. on a cropped sensor the issues of light fall off may not be as extreme but the lens greatly improves with the CF. I believe the CF is still manufactured. The lens is not. Outer threads should be 72mm as I recall.
Paul C