The R17 is a HW power rectifier, the special quality version is the 6157.
This rectifier has to be operated with a minimum of 50 Ohms of series resistance to limit the peak current. The maximum reservoir capacitor is given as 32 µF. The anode is taken to the top cap for additional isolation of the high voltage across the small envelope.
The working anode space is a small cylinder with long side flanges to dissipate the heat. As the valve is small the ends of the flanges are opened out. If seen from above the anode would be a capital I in shape. The bright rod in the centre is a support.
The support rods either side. The image shows the ends of one side of the flanges bent outwards. Normally the inner surface of an anode is left bright here they have been coated to radiate more heat.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 62 mm tall.