The SS210RC from Six Sixty is a Mullard PM1A. This triode was designed for Resistance Capacitance Coupled (RCC) applications.
In this image the paper label has the Six Sixty logo and the words Glowless Valves. Other text on the label is the limited license details. Not easily visible in this picture is the same logo etched into the glass. Our other SS210RC has clearer lettering.
The logo on the glass a little clearer. The logo is contained in a circle with Trade one side and Mark the other.
The box anode lies horizontally in the envelope. The image also shows more of the faint etched lettering that is on the glass. Some of the wording is: Covered by various patents, 802N & SS210RC under the logo. On the other side is the BVA logo.
The balloon envelope is 41 mm in diameter, and excluding the B4 base pins is 83 mm tall.
References: Datasheet & 1043. Type SS210RC was first introduced in 1928.