I may (finally!) be coming into a 4x5 camera and able to use the Polaroid 500 holder I've had for a couple years, possibly including using Type 55.

However, having recently shot a bunch of Type 667, I've noted that the paper negatives display a Sabbatier effect that begins to appear within a few seconds of peeling off the print (and exposing the negative surface, still coated with developer gel, to light and air); areas that started out white (presumably with undeveloped halide) turn gray in exactly the same patterns I'd expect in a developer-wet print exposed to light. I presume this doesn't happen with Type 55, and I'm curious how the process differs between the two to prevent undeveloped halide in the negative, still wet with developer, from darkening -- or, since it's this silver that should produce the print, what's turning dark in the 667 paper negative, even in areas that are full max black in the print? Or would the 667 negative lose or not develop this Sabbatier appearance if it were promptly put into a clearing bath like that recommended for T55 negatives?

I've read lots of descriptions of how the Polaroid B&W print process works (I read the first one in about 1966), but this detail seems to be left out, because I've never read a description that includes the difference between the common print films and the pos/neg materials. Not really critical to anything (the stuff works if used as recommended, which is the main point) other than my curiosity...