The SP610/B is a valve from the Cosmos 1926-8 six volt range. The SP610/RR is an audio output valve but we do not have ratings for this version. In other ranges the /B suffix was reserved for resistance/capacitance coupled voltage amplifier stages. Possibly the SP610/B could have acted as a driver for the /RR output valve.
The etched lettering on the balloon dome includes the type designation the brand name Cosmos and the MetroVick and BVA logos.
The box anode is wide and tall but has a small front to back dimension.
The electrodes close-up. Four strands of oxide coated filament pass through the grid.
The balloon envelope is 50 mm in diameter, and excluding the B4 base pins is 98 mm tall.
Reference: 1043. Type SP610/B was first introduced in 1928.