The CV491 is a low μ double triode with separate cathodes. An amplification factor of 17 is quoted. The prototype was the 12AU7.
We were surprised at the very low CV number, suggesting availability during WWII but we know that B9A based valves were not made in the UK until the late 1940s.
The KB/FD identifies this valve as being made to specification K1001 or K1006, the B denotes qualification by a UK authority and the FD identifies the maker as STC, Rochester.