Am I right in thinking that there's a difference? And that this is it?
Chalky Paper: Older stamps were occasionally printed on paper treated with a chalk solution. The paper, that is, was first treated and the ink then applied on top of that chalky substance. Thus, the ink did not sink into the paper fibers. This was done to prevent cleaning and reuse of stamps: a promising idea, insofar as contact with any cleaning solution, including water, would wash off the ink. In practice the ink was usually just compromised, leaving the stamp looking severely faded or badly discolored. In addition, some countries, e.g., Russia, treated paper with chalk in an effort to prevent stamps from curling, a common problem with the early use of gum.
Chalk Surfaced Paper: Modern stamps are often treated with a chalk or clay substance, not in order to prevent reuse or curling, but to produce a clean and shiny appearance. In this case, the ink is first applied to the paper; the chalk is applied afterward, i.e., on top of the ink. Soaking may affect the shininess, but not the ink.
Please correct me if this is inaccurate or simply plain wrong.