From 1926 onwards the Cosmos (Metrovick) company had pioneered a novel form of construction in which the grid had a single but very rigid support rod and was surrounded by the anode on 3 sides only.
This 'Shortpath' arrangement gave high gm without causing high sensitivity to microphony (a distressing problem affecting many high-gain valves of the later 1920s). The AC/G exploited Shortpath construction but its very compact anode limited its dissipation rating, bearing in mind that there was already 4 W of heater power to get out somehow.
In late 1928 Metrovick and Ediswan combined under the Mazda label. The AC/G was uprated by extending the anode 'wings', and was renamed the AC/HL. The characteristic style is shown in the photo of the AC/HL.
In much the same way the Cosmos AC/R became the Mazda AC/P and the AC/S became the AC/SG.
The Cosmos originals were introduced before the B5 base became standard and were therefore fitted with special adapters to enable the 5-pin valves to fit into standard B4 sockets. This arrangement was retained on the earliest Mazda versions of the AC/HL but from about 1930 onwards the B5 base was fitted.