Ted - that big tube image I just posted was a huge mercury-arc rectifier. Google "mercury-arc rectifier" and see if your tube shows up in image search.
Andy - the metallic shine on top is caused by a "getter". A getter is... oh... wait - has this been answered already?
Vintage military German WWII valves.
These are the strangest I have ever seen.......
German vaccum tubes, WWII vintage. photo - Rolf photos at pbase.com
They were inserted into the socket with the brass
pin on top first. I assume that was the connection to the plate (anode).
These were removed from German equipment left in Norway after the occupation.