[jerryhawthorne]

John, to expand a bit on Morzh's comment, if your referring to the black Philco bakelite caps they generally have a lug on the end the mounting screw goes through. This lug would be chassis ground. Components inside the tub often require one end to ground and this is used for that connection. Should you find a can without a lug on that end (I don't recall seeing any) it just means that the component(s) inside do not need to be connected to ground thus the lug was left off.
Make sense? Chuck's Philco repairbench site will provide a listing of the various tub caps listed by the number etched in their sides. It provides the value(s) of the internals and their connection point.
Chuck's site: http://www.philcorepairbench.com/bblokcap.htm
Jerry