Cossor 63-SPT Amplifier tube
The 63SPT was A C Cossor's type designation number for the EF50s that it made. The valve was so important during WWII that several manufacturers were involved although the design was a breakthrough for Philips in The Netherlands.
The wide metal tube envelope is 32 mm in diameter and, excluding the B9A base pins, is 60 mm tall.
The 63SPT was first introduced in 1939.