I can tell you, they work wonderfully!
My uncle had a rifle very similar to this one with an adjustable cheek/comb and hooks. These were shaped more like ice hooks but still recessed neatly into the forend.
My impression is that they were for use in forests of big trees and when anchored against the side of a tree is very stable and allows almost complete concealment of the hunter. They both popped out on each side - then you folded the side you didn't want out back in to the catch. I guess you could simply hold the one you didn't want in while you released the other. Being a kid I didn't think about that at the time and of course in a real hunting situation you would do it very quietly anyway.