Flip-top simply means that the razor's top cover that goes over the blade "flips" up to allow changing of blades. Here you can see two examples of flip-top heads. The black and silver model is a push-button flip-top whereas the gold razor's head is "flipped" up by rotating the base of the handle, similar to the way many double-door headed razors work. Flip-top heads are almost never found on razors other than those manufactured by GEM.
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I call theses razors sandwich headed because the blade is sandwiched between two solid pieces of metal, the top (picture, left) and the bottom (picture, middle), thus a razor blade sandwich. This "sandwich head" was the attached to the handle by screwing the handle (picture, right) onto the threaded stud which passes though a hole in the blade and the bottom piece.
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