Thank you Mark, I guess you are right, that is a likely explanation. To confirm that I guess I can, as suggested, try to contact Mark Vieira, but I think your guess is correct. I would need now to find technical info (spectrograms and sensitometric curves) of Super-X, Panchro Press and Super Sensitive Pan, which I can't find in any kodak book (my oldest one is from 1939, which seems "too modern"). Maybe it is better to open a new topic for that.