The 9002 is a UHF receiving triode for use as a Class A amplifier. The electrode assembly is very small, of the same scale as an acorn valve and very likely the electrodes of a 955 acorn valve in a B7G envelope.
B7G valves were make in the USA in the early 1940's but not made in the UK until 1947 or so.
The etched lettering is very difficult to see but is present as 9002 within an outer octagon - possibly an RCA valve.
The front to back dimension of the anode is around 1.5 mm.
The grid is placed close to the cathode as can be seen from the supports above the top mica.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 37 mm tall.
Reference: Data-sheet. Type 9002 was first introduced in 1943.