This is a used and not in currently working condition Bendix Treadle-Vac power brake booster taken off of a 1957 Pontiac 4 door Chieftain sedan.  The interchange I found shows it will fit the '55-'57 Pontiacs.  As you can see in the pictures, this one needs to be completely taken apart and rebuilt (or used as a core to buy an already rebuilt one).  Some other condition issues to note are that it is missing it small filter that sits on top (see picture #8) and it has one bolt broke off in mounting nuts (see picture #10).  The part number on the front looks to be 5454956 (see picture #6), but it is difficult to be sure if that is correct.  It did have some oil run out of it when I took it off and turned it over, so maybe that protected the internals.  But, I haven't had it apart to know for sure.  Please contact me if you have any questions and thanks for looking.