The P552/1e is the higher voltage version of the P535/1E pulse modulator. The P552/1E was designed for use in airborne radar modulators and the heater rating was for use with the standard 24 Volt aircraft batteries where the voltage could vary from 22 to 28.5 volts. The anode voltage was up to 20,000 Volts and the peak pulse current was 15 Amps. The four parallel cathodes being required for sufficient electron production.
The main visual difference between the P535/1E and the P552/1E is the top cap connector. Here there is a saucer around the base of the connection to reduce corona discharge from the 20 kV anode voltage. Also note that the screen grid does not have a radiating fin fitted.
The four cathodes are in parallel and the grid wires are gold plated.
The wide glass tube envelope is 64 mm in diameter, and excluding the B4A base pins is 139 mm tall.
Reference: Datasheet. Type P552/1E was first introduced in 1946.