Plate where the filler cap is, is different. That threw me off. It's not flat.55-57 Tredle Vac booster...See link:http://www.trifive.com/garage/55 Chevy Assembly Manual/412-2.gif
The caps can be swapped put, The big difference is the vac connection. The 55-57 is on the side off the booster. The one in the pics it is on the rear indicating an "under the floor" application. I don't think the 55-57 unit has a bleeder either. :flag6:Your right (of course) Bruce....Just looked at the one I took off the 55, it has the flat cap.