Ferranti 1B23 TR Cell, serial number Y3436. Designed to work in an external cavity, frequency range 0.9 to 1.2GHz, at 450kW. The mixture used is hydrogen and water vapour. With a suitable modification kit, the 729A replaced the 702B.

Western Electric produce the same as 729A.

Also marked on the valve is FE 273 and F.LTD E.297.

Overall length is 72mm, 19mm across the glass and 30mm across the disc.