The UAF42 is an early all glass valve introduced in 1948 together with the EAF41 and consists of a single diode and a variable μ RF amplifier pentode in a single envelope.
The locating pip can be seen in the main image. The glass is wider at the base but not formed into an explicit replacement for the original metal skirt.
The valve is screened and the envelope has the moulded glass pip for pin registration in the metal skirt of the base. The first B8A valves had metal skirts fitted with built in locating pips and spigots to guide the valve into its base.
By the early 1950s the B8A valves were being phased out in favour of B9A and B7G based valves.
The slotted screen has the mica discs inserted within it - unusual.
The open pitch of the suppressor grid can be seen through the slots.
The thin glass tube envelope is 21 mm in diameter and, excluding the B8A base pins, is 54 mm tall.
References: Data-sheet & 1040. Type UAF42 was first introduced in 1948.