The Philips 21- "Key Base" tubes were initially designed in 1940 with the 9-pin base B9G like the EF50. The later series production from 1942 on however was changed to the 8-pin Locktal-base B8G designed by Sylvania.
Instead of the later UBL21, the CABL21 was planned, containing 3 diodes especially for the then modern 3 diode- detector- circuit.
