The AC/TP is a triode pentode that was designed as a frequency changer for broadcast radio reception.
The black markings on the metalisation indicate a model from the later end of production, possibly 1940s
The triode oscillator would have to generate 3 Volts peak to peak. The pentode, operating in a non linear mode, would be the mixer. The input signal would be applied to the top cap of the valve and this would provide some isolation from the oscillator circuit.
The outer metalisation hides the inner construction.
The balloon envelope is 43 mm in diameter and, excluding the B9 base pins, is 106 mm tall.
References: Data-sheet, 1040 & 1043. Type AC/TP was first introduced in 1934.