I have a reprint of an old RCA transmitting tube manual, which lists a type 5561. I suspect it would be the same or very similar to your Westinghouse tube. It's listed as a half-wave mercury-vapor rectifier, to be used in the power supply of transmitting and industrial equipment. It has an ST-30 outline (11.25 inches tall from bottom of pins to top of plate-cap; 3-13/16" across the widest point of the shoulder. Pin 1 is nc; heater is between pins 2 and 3; cathode is between (sic) pins 2 and 4. There are various voltage and current specs, which are VERY large. Let me know if you'd like further details.
Regards,
-- Al
Regards,
-- Al