Anyone recognise this header tank?

This coolant header tank is similar in size to a Shackleton, but the connections are in different places, and unusually, it is made either from stainless steel or inconel.
It is a hollow tank without internal steam-separators, which means these would have been integral with the coolant outlets on the cylinder blocks. This system was only used as far as I am aware on the Shack, Balliol and Athena
The drawing number on it reads 11-E-12???
Nobody at Rolls-Royce recognised it, but it is certainly British, and there were not many other types of liquid-cooled engines. Sabre and Nomad being two.
It has the standard Teddington relief valve and AM filler cap fitted.

Any ideas?

Pete