The 4687 is a neon voltage stabiliser tube. The 4687A variant is on a B4 base. Both have identical electrical characteristics.
The striking voltage is 130 Volts and the regulation is 6 volts across the full current range.
The top view. The interference fit mica holds the concentric cylinders firmly in position. The geometery of the electrodes governs the characteristics.
This view from the pinch end shows that the cylinders are formed from rolled sheet. The accuracy of the cylinder forming and fixing seems less than perfect and suggests that a spread of characteristics would be expected from tube to tube.
The wide glass tube envelope is 26 mm in diameter and, excluding the Ct8 base pins, is 86 mm tall.
References: Datasheet & 1040. Type 4687 was first introduced in 1937.