The 715C is a 60 watt dissipation tetrode designed for high voltage pulse modulator service. Similar to the 715A and 715B. The first example was made by General Electronics and the second by Lewis & Kaufman. The third example was made by Western Electric and seems to be a standard 5D21. Where the tube number is usually printed, the glass has been sand blasted to remove the marking and the base is stamped JAN-715C. My explanation is, that in order to fill a contract for the earlier 715C, WE remarked some 5D21 tubes which would be interchangable. Only the plate attachment might be a problem.
Length = 5.50" Diameter = 2.37"
Max voltage = 15 kv peak
Max current = 15 amps peak
Fil voltage = 26.5
Fil current = 2.1 amps