As far as I know, there isn't a dedicated 35mm mixing box. There's a 4x5" and a 6x9 cm one. The 6x9 one will give about 1 stop more light through 35mm film than the 4x5 mixing box will.
I think the 35mm box gives perhaps about a stop more light than the 120 and maybe 2 stops more than 5x4. Have never measured the difference but if you have the 35mm box you'll get far shorter exposures than 5x4. Ilford used to sell all the three mixing boxes plus the adapter plates for many different enlargers for £15 a set. Wish they still did.
If you contact KHB Photografix, they may have a 35mm mixing box. They were the only ones in the world who had a 5x4 mixing box for the 400 mg head I just got with my 504...Evan Clarke
Is there any advantage to be gained by using the appropriate box? I have 4x5, 6x7 and 35mm, but on advice from someone on this list who had used the Ilford MG head in a professional lab (ie not enough time to fart about changing boxes) I use the 4x5 most of the time for all gauges.
My only thought is that if I were to burn out the 4x5 I may not be able to use the 6x7, nor the 35mm boxes for 4x5 printing and as has been posted, they are rare.
I did a quick comparison for you on my 504 with the Dichromat head and Colour 500 computer. Using 20M and 20Y filtration the time difference with the 120 and 4x5 mixing boxes were as follows: 4x5 @ 9.2s 120 @ 5.2s That gives you a good idea of what to expect. I used to have a full service lab and I don't know of any exclusive 35mm mixing box ever made for the 504 or other Devere models.
The only advantage is that you get more light, which could be useful for large prints or with really dense negatives.
Brighter isn't necessarily better, though. You want to use your enlarging lens at F4-f8 for maximum sharpness. If you have to stop down to F11 or, especially, F16, you'll be losing sharpness to diffraction.
Hi Bob, I have three light mixing boxes for my DeVere 504, 35mm, 69 and 45, however I only use the 45 for all formats. But to answer your question, yes as others have said, you will get a brighter baseboard image with the 35 mixing box and 35mm negatives.
When using a 504 I used the 5x4 mixing box for all sizes, When I used a 10 x 8 enlarger I would change boxes for smaller formats as otherwise you had very long exposure times.