B&W reversal cannot be done in a monobath -- but isn't necessary for a tintype. The "reversal" occurs when you fix away the milky halide, leaving the silver image. That silver looks dark against the halide, but becomes the light areas against the black substrate as the halide is dissolved.
I wasn't confused on that but I was confused if that's what they were actually doing in 1870's with dry plates. The postcards are from 40 years later.
Messaging with jnantz that sounds like they were doing exactly that in the 1870's, with specialized developers for making appropriately week images on gelatin emulsions. The developers were quite proprietary but that was the premise.
Language doesn't help here of course. The positive produced by 1) producing a weak negative against a back background 2) bleaching out the negative and creating a positive from what remains (slide film) 3) creating a special emulsion that essentially solarizes on exposure like Harmon direct. 4) dye destruction positives like cibachrome.
Are very different. I think Polaroid is different too.
Afgan box camera by my understanding were originally paper negatives with standard developer and fixer trays. The reversal was made by subsequently photographing the resulting paper negative with the same camera (also making left / right correct).
Just wondering, I wonder if a 2 bath true b&w reversal is possible. Bath 1 develops and bleaches. Bath 2 fogs and fixes. It'd be a nasty chemical cocktail because of the dichromate bleach so no one would likely commercially produce it, but it'd be interesting if it's even possible.