The VCR138 cathode ray tube is a WWII classic. The base is 12 side contacts and fits into a special base. The screen is round and has a 3 inch viewing diameter. The phosphor is probably P1 and designed to give a medium persistence green trace. The VCR138 was introduced in early 1940 for use in airborne radar indicators and soon after introduction the VCR138A was developed with an improved screen.
The operating voltages are arranged to give the final anode at deflection plate potential of, say, 200 Volts. These leaves the cathode at about minus 1,000 Volts. The heater cathode insulation is poor and the 4.0V heater at 1 Amp would have to be supplied by a separate transformer winding. The deflection is electrostatic. In normal practice A1, A3 and the Aquadag colloidal graphite internal coating are all strapped together.
Our example is marked with a crown and the letters AM for Air Ministry.
The base connections looking into the base. Pin one goes to the grid.
The form of the driving circuit for this age of cathode-ray tube. In this case the use is a simple oscilloscope.
The end window envelope neck is 58 mm in diameter and, including the B12D base pins, is 337 mm tall.
References: datasheet & Radio Constructor article. Type VCR138 was first introduced in 1940.