I have had the "pleasure" of paying to have several posters removed from masonite.
First, there is no restorer who does this more than once, because it takes FOREVER to do, and whatever they charge is not enough.
Second, the only way to remove it is to shave the masonite from the back, layer by layer, until you have a thin film of it left on the back, and then the most careful shaving of that thin film (I don't believe anyone has found a solvent to separate it without damaging the poster).
Since the poster looks fine, and since it will look fine decades from now, just leave it as it is (this one, and any others you come across).