The ST63 is a six volt 'Super Power' directly heated audio triode that dates from 1926.
John Scott-Taggart was a famous designer of radio circuits and some valves. The valves used in some of his designs carried his own branding and identification. However, the valves were made by Mullard and the ST63 is the the Mullard PM256.
The faint etching on the envelope. Within the outer shape are the letters ST and the word Radio above and Valves below.
The horizontal anode box. The visible etching shows the 6.0 V 0.25 Amp filament rating and the words Super Power.
Another view of the etching.
The balloon envelope is 45 mm in diameter, and excluding the B4 base pins is 93 mm tall.
References: Datasheet & 1043. Type ST63 was first introduced in 1926.