But how does a B/W silver emulsion survive intact through the bleach and fixer of a colour negative process? It was not a dye forming layer. In some references to the film the B/W emulsion mask layer is stated to have been a slow blue sensitive emulsion, (Progress in Photography 1940-50 p80) My theory is:- Say the B/W mask layer was twice the sped of Kodacolor film, then 25 ASA. Mask layer is 50ASA. The bleach and fixer would remove all the silver from the three dye forming layers, but leave enough silver in the mask layer to make a weak positive image when exposed to white light. The chemicals would leave enough silver in the B/W mask layer because it takes longer to fix a fast emulsion (mask layer) than a slow one (dye forming layers). As the mask was blue sensitive, it would have been unlikely to have been affected by camera exposure on account of the yellow filter.After C-22 processing, the film might have been handled in a Bromide safelight (Wratten No. OA, in those days) until exposed and B/W developed. If anyone can confirm that my theories are correct, (or maybe the film was processed in a different way) I would be very grateful.