The ST11 is a voltage stabiliser based on the 7475.
The Osram paper label.
The active electrods can be seen through the top of the glass. The neon discharge will be between the top disc and the larger lower cylinder. The CV specification requires that the glow be visible through the top of the valve in order that the ST11 can be used as an indicator.
The wide glass tube envelope is 29 mm in diameter, and excluding the B4 base pins is 67 mm tall.