The 210LF from Six Sixty is a Mullard PM1LF branded for Six Sixty.
The PM1LFwas part of the first set of Philips-Mullard valves. The design of the PM1LF AF triode, dates from 1926 and the Six Sixty version arrived in 1928.
The dome carries the identification etched into the surface of the glass.
The box anode is mounted horizontally in the upper part of the envelope. The fine grid wires can be seen at either side of the bright anode.
The balloon envelope is 41 mm in diameter and, excluding the B4 base pins, is 78 mm tall.
References: 1043. Type SS210LF was first introduced in 1928. See also1928 adverts.