Cinderella: Stamp images by photo gelatin.

GB 1950 Waterlow.
http://cjoint.com/data/exiU062hcv.htm

The collotype process was used between about 1870 and 1920. A glass plate 
was coated with sensitised gelatin and exposed under a negative. Light 
passed through the negative would harden the gelatin on the glass plate.
The 
unexposed gelatin would absorb the water when washed and the exposed would

repel it. The washed glass plate would be coated with ink, adhering to the

exposed gelatin and printed onto fine paper.

"Collotype: seldom used with stamps"

Anybody aware which stamps may have used this process?