The 13D9 is a mystery valve in that no reference to it can be found in the available literature or on the internet. However, it is widely reported to be a special quality ECC81. The standard special quality or trustworthy ECC81 from Brimar is the 6060. It is possible that the two are the same but the 6060 is the standard American designation and the 13D9 follows the Brimar naming convention.
This is a UK made valve.
The box anode sections and copper grid supports. The getter ring support consisting only of a short bracket affixed to the upper mica; a corresponding bracket is affixed on the underside of the lower mica which has "wings" that connect to the common side of both heaters.
Thanks to R J Sutherland for the excellent images and information.
The thin glass tube envelope is 21 mm in diameter and excluding the B9A base pins, is 47 mm long.