The AC/R from Cosmos (Metro Vick) was an early AC indirectly heated valve and was released at the September exhibition in 1927 as the advert declares.
The aim of the design was to enable people to take advantage of the newly installed electricity in their homes to replace the battery valves in their wireless set with these AC valves with the minimum of alteration to the set. To this end the base was a modified B4 where pins 1 to 3 were as the B4 pattern and designed to plug into a B4 valve-holder and pin four was replaced by a pair of studs. The valve was supplied with an adapter that slid over the longer than normal base pins and made contact with the studs and brought the heater out to a twisted pair of wires for separate connection to the AC heater supply. This adapter can be seen with the AC/G exhibit.
The valves released in 1927 were the AC/G and this AC/R output triode.
These AC valves featured a revolutionary dull-emitter cathode thereby reducing the possible hum induced into the mechanism by heating the cathode directly. In production the heater was dipped in liquid porcelain clay and fired to produce a hard, very thin insulation. The tubular cathode was a ground-breaking 1 mm in diameter. It would be five years before this technique was beaten for rapid warm-up time, efficiency and low operating temperature.
The studs that replace pin 4 and on the envelope is the etched Metrovick logo, Cosmos name, short-path and Type designation.
Here is the etched lettering in close-up. The lowest line says Red Spot.
The BVA logo and electrodes. The top uses a glass bead to hold the supports securely and insulated from one another.
The balloon envelope is 60 mm in diameter, and excluding the special base pins is 124 mm tall.
References: Advert, Emil Dudek (Vintage Technology website) & 1043 Type AC/R was first introduced in 1927.