why has the title been changed from "covers" (which is what they were) to "stamp" which is a less specific form of what they were? Why are we not just using the article title?
Because most people have no idea what covers are, and unless you collect postage stamps the difference means nothing to you. If you'd asked me before I met a friend who collects, I would have assumed that using a stamp would render it worthless.
The article's talk page actually has a similar discussion over whether to call them stamps or covers in the title, so although they come down on your side its hardly a given.
in my 40+ years of living in english speaking countries and sending thousand of pieces of postal mail over that time, i've never heard the phrase 'postal cover'. 'postal' is the only clue that indicates it's ... to do with 'postal' stuff.
The article's talk page actually has a similar discussion over whether to call them stamps or covers in the title, so although they come down on your side its hardly a given.