This photocell has a coating on the inside of the envelope that is obscured by a semi-circular electrode in front of it. The coating looks to be the anode with the internal electrode being the light sensitive component.
The Type designation starts with the letters IR and this probably indicates the range of wavelengths that this was designed to operate in.
The paper label carries the serial number of 561 and the maximum anode voltage of 100 Volts.
There are three electrodes present. The silvered coating connected to the top cap. The photo-cathode and a ring structure. Note also the wire coming out of the base cap - purpose?
The ring structure connects to the pinch supports by a single turn coil on either side.
The side window envelope is 30 mm in diameter and, excluding the B4 base pins, is 102 mm tall.
