In seeking a working definition for the term radical pamphlet, I would suggest beginning with defiinitions of the two components of the phrase. Pamphlet may be the simpler of the two to define - according to a few dictionaries at hand, a pamphlet is generally a short printed work not bound into a cover, though it could be stitched or stapled. Pamphlets are typically single items or parts of a limited series, rather than ongoing publications such as magazines. They would usually have a single subject focus and/or advocate for a particular point of view, rather than being a creative expression of an individual "as an individual" (what we would now call a zine).
Radical, on the other hand, is a fairly slippery term. Derived from the Latin radix, or radicis, meaning root, in math and science contexts it referrs to the root or foundation - getting to the bottom of things, so to speak. In the social/political realm, which is the usage to which your question refers, it has taken on the meaning of "one who carries his theories or convictions to their furthest application; an extermist." (Funk & Wagnell's New International Dictionary) In a way this seems a contrast to the root meaning. The concept seems to be that in getting to the root of social/political issues, one comes to conclusions and courses of action that are outside the "mainstream" status quo.
As you work on your working definition and your subject areas, you might want to consider that there are "radicals" of many persuasions. The terms you list all seem to come from what (for lack of more precise terms) would be termed left politics. You will also find many pamphlets produced by radicals from the right. Consider for example the right-libertarian John Birch Society, a group that produced a great many pamphlets (see this collection inventory at PublicEye.org for a list of some titles).
The use of the term in our name, of course, does indicate an overall left orientation, though there is no unifying political ideology at work. We like to think that as librarians were are inclined to advocate research to get to the root of things, and that the name is based on that older definition of radical. (Plus it has a catchy ring to it.)