I have a source for a complete working 500H head, I am tempted...
When I print 4x5 on my 45MXT with the Aristo Cold light, the light source has to be nearly touching the glass carrier to get even illumination. So there is not much room in there to engineer a 6" MG filter drawer..in fact there is none as it is recessed. The other big problem with that is that if you split grade like I do, you are pretty much assured to stir up dust that will land on the glass carrier as you go back and forth swapping filters out.
I am pretty sure the 3x3 under lens filters are just as thin as the 3 1/2 x 3 1/2 and Ilford does state on the box that they can be used under the lens. The filters sit perfectly flat in the Cokin holder, FYI. I also did a test awhile back at 11x14 with my regular Rodagon 150 with and without the filter in front, no difference in overall image sharpness..
Maybe when I print 20 x 24 I will test it again with the new filters and see what comes up. While this is a great craft, there is just not a lot out there for pro level printers in terms of cold light splitgrade, kinda stinks....
If anyone has a viable solution for me to print cold light split grade without putting MG filters under the lens, I am all ears...
Otherwise:
Ctein in "Post Exposure" has this to say: "According to my tests, this makes no difference. I set up a high resolution test target with my 55mm Computar lens at optimum aperture and examined the projected aeriel image with no filter under the lens, with modern thin filters under the lens and older cast-plastic filters under the lens. In all cases, I could see a clean 320 lp/mm in the center of the field and more than 280 lp/mm at the corners. I could not convince myself that I saw any degradation in image quality with the filters in place, no matter how hard I looked. As long as your VC filters are not scratched enough to create serious flare, I can see no reason for avoiding below-the-lens filters."