The TH233 is a triode heptode frequency changer for 200 mA series heater chains. The TH2321 is the same basic design but with higher internal capacitances and the older B7 base.
The reverse is plain zinc paint.
Part of the internal structure seen through the gap in the zinc paint. The dark cylinder at the top is the heptode and the grid that can be seen is of the triode oscillator section that sits below the mixer. The triode anode is left bright.
The classic envelope is 32 mm in diameter, and excluding the MO base pins is 92 mm tall.
References: Data-sheet, 1040 & 1043. Type TH233 was first introduced in 1939. See also1939 adverts.
