The UV217C or 38117 "Rectron" is a half wave, high voltage rectifier. They are similar to the 217/A types, but have a top plate cap. By WWII the structure of the tube had been strengthened and the "UV" dropped. The first example is an RCA and the second was made by Westinghouse ca 1941. The third was made by Lewis Electronics who used uranium glass in the filament press.
Length = 8.25" Diameter = 2.31"
Max voltage = 7500 PIV
Max current = .6 amps
Fil voltage = 10
Fil current = 3.25