The X66 is a selected X65 and also known as CV1099. It is a triode hexode mixer or frequency changer. The ellipse with the Type designation is typical M-OV.
This exhibit is marked for ground use only and was supplied to the RAF.
Under the overprint is the original etched identification of VR99 and the old stores code of 10E/277.
Also under the overprint is the AM initials (Air Ministry) and the Crown.
The classic envelope is 38 mm in diameter, and excluding the base pins is 99 mm tall.
References: Data-sheet & 1043. Type X66 was first introduced in 1940. See also1940 adverts.