The 6P26 is an output valve, for frame timebase or audio. It is similar to the 6P25 but has reduced heater power and, therefore, probably a lower output. But also it is in a tubular envelope and could just be a more modern implementation of the 6P25 design.
The lower mica with copper control grid supports and tape connections.
Below the lower mica is a screen. The anode has side flanges for fixing and increased surface area.
One side of the 6P26 box showing the price and referencing re-sale price maintenance that was outlawed in the UK in 1956 by the Restrictive Trade Practices Act.
The wide glass tube envelope is 29 mm in diameter and, excluding the IO base pins, is 77 mm tall.