Well, to be clear here, no one has explicit confirmation that Polaroid Type 55 is Panatomic X. I don't even know that. I do know that Kodak was involved in making all instant products happen as Land didn't know how to do it himself, nor did any of his people. So Kodak taught them. And, since I know how Kodak works, I doubt if they used one of their own product formulas for the teaching process and for handing over to a 'competitor'.
Just FYI, Kodak used to do R&D for Ansco as well, when they got stuck on problems. This does not mean that Anscochrome was E1 Ektachrome by any means.
PE
Very true Ron,
The ID of 55's neg is only a guess, but whatever it is, there isn't anything else like it, except Panatomic X, if that isn't what it is.


