The CV27 is a directly heated power triode with a thoriated tungsten filament based on the 4357A. And this exhibit carries both designations and thus the CV spec matches the commercial valve. The original design is American and was Type 357A from Western Electric. ITT was the parent of STC and when STC made valves in the UK to the American design a 4 was added as a prefix to the Type designation.
The attached PDF gives the basic CV details but here can be found much fuller data of the B variant from Machlett.
The design is for power amplification at RF up to 100 MHz or as a class B modulator. For telegraphy the power output in class C can reach 1 kW but for use as an AM or FM transmitter in Class C and output of 500 Watts can be expected.
The STC logo and 4357A designation.
The four thoriated filament wires look to have tension springs at the top but the fixings could be rigid.
The original Western Electric 357A.
Thanks to Andrew Horner for sending the images that I supplemented with 357A images from an ebay listing.
The wide glass tube envelope is 130 mm in diameter, and excluding the base pins is 165 mm tall.