No it was Agfa APX 400 and TMAX, it's the anti-halation dye added to the back of the film intended to increase contrast by stopping light been reflected back from / by the substrate. In some films it forms a dark red cloud when combined with the thiosulphate in the fixer. ( IIRC it's a long time since I did photoscience ) Sorry my first reply was not so informative but I have been pondering it all morning.