Michael, I got out my old 1965 Kodak Pub B-3 on Wratten filters and found the spectral-transmittance curve for the Magenta #32 (aka Minus Green). It transmits blue and only a miniscule trace of green, but it also transmits a good bit of orange-to-red starting at ~610nm. So the filter technology has long existed to make such a filter, and although Ilford's #5 is likely proprietary, it could be very similar to the #32 in order to get the blue/green mix they want. But they might have to live with the bit of orange that comes with it. You see the orange, but the paper is blind to it.
Originally Posted by Michael R 1974
Just a thought.