[Amps] Can someone ID this tube and its use ?

[John Lyles]

>From the photos, it looked like the glass was stained brown. Is this the case or just the camera/lighting? If so, then it is probably a tube that (like 4PR1000) ran very high voltage, and the darkening is from continuous Xray bombardment. It would have run under oil in that condition, most likely. Is there any scent of oil or residue under the base? 

Jim Tonne, are you sure about Tom Yingst? He was at RCA/Lancaster for years during the development of many tubes, but I thought his tenure at Eimac was shorter and that the Y designations preceeded him there. There were plenty of Y numbers for their developmental and special tubes that were for sole use by a particular customer also. 
73


John 
K5PRO