Is this the spacer that Marty mentioned? It's been in my garage for years.
Neil
Neil, I think that may be part of the spacer. I think that part is maybe a plastic gasket-type part that fits in the aluminum spacer. Here is a link to a Midland on ebay which I think has the spacer that we're talking about. In one of the pics you can see the part that you have is part of that larger metal spacer.
http://www.ebay.com/itm/1967-1968-1969-Mustang-Shelby-Boss-Cougar-Midland-Power-Brake-Booster-/181159432573?pt=Vintage_Car_Truck_Parts_Accessories&hash=item2a2df1917d&vxp=mtr