15 years ago I was used to buy truckloads of test and communication
equipment from Johns Radio in Bradford (now he is in Batley).
He had a lot of equipment from the closure of STC R&D labs. I
acquired many valve handbooks: AEI, GEC, STC, RCA, about 20 books.
I dug and found some data on a similar tetrode, possibly a newer
version of 4B/55. I have 4B/550E, 4B/551B, 4B/551BD. It is in Volume
2B, Components, of S.T.C.
Please find the scan in www.spinelectronics.eu/ftp/STC_tetrodes.pdf
Petr, do they look similar?
73 - Marco IK1ODO