The 85A1 voltage stabiliser, from 1948, will strike at 125 volts and is useful for output currents of between 1 - 8 mA. The preferred operating point is 4.5 plus or minus 0.2 mA this indicates a design for a reference tube and not a direct stabiliser.
The envelope is opaque from all angles. The additional codes are date and location codes.
The base is a standard B8B but is unusual in that only three pins are present. I suspect that in practice this valve would have been held in place with a retaining clamp.
The wide glass tube envelope is 27 mm in diameter and, excluding the B8B base pins, is 62 mm tall.
References: Data-sheet & 1040. Type 85A1 was first introduced in 1948.