According to my Sinar literature from the 70’s when I bought this lens, there is no need for the center filter. My 8x10 color trannies - when I used to shoot color - showed no problems at the edges at infinity focus. As for filters, I bought a bunch of 4x4 gels as well as Series IX glass filters before their prices went into orbit. For this lens I found that I had to use the gels in front. I’ve never used filters in the rear, but as you probably already know, rear filters should be gels anyway as the thickness of glass MAY introduce some loss of image quality (compared to gels). Looking at my lens it appears that 3x3’s would be too small for behind the lens. So you may as well go with 4x4 gels for either location. Here is a rear gel holder that may work for this lens, but check it for fit:

http://www.calumetphoto.com/item/XB6560/