Mazda introduced the AC/Pen in 1930 and it was the first technically successful indirectly-heated power pentode capable of enough output to drive a moving-coil loudspeaker at good volume. Here we have an AC/Pen made by Arctron. It must date to 1933 or later as it has the B7 base cap.
The AC/PEN and 250V cab be read but the Arctron name is very feint.
Looking up into the envelope the anode can be seen to have a cylindrical working cavity and four flanges. The inner coating on the glass makes this difficult to see.
An Arctron box courtesy of G3YNH, as seen on his website.
The classic envelope is 58 mm in diameter and excluding the B5 base pins, is 112 mm tall.
References: Datasheet & 1043. Type AC/Pen was first introduced in 1930.