This experimental mic was made in very small numbers. It utilised two magnets and was coded PGD - Pressure Gradient Double. It was produced at the same time as the PGS but never manufactured commercially.
It had the same output level as the AXBT, weighed half as much, was about 7 by 2½ inches and had a ribbon similar in size to that of the PGS. The increased sensitivity compared with the PGS was achieved by mounting a horseshoe magnet at each end of the ribbon. This made the PGD more bulky than the PGS with no advantage except 3 or 4dB greater output.
The external casing with its flat indents to replace the internal fibre plates of the PGS probably developed into the case design of the 4038.
There was one in use in BH in the 50s and 60s in the original 3B and later in the Type B 3C that resulted from knocking together the 3B and 3C. The documentary film "This is the BBC" shows that a PGD was used in Light Continuity in the late 1950s. The pictured mic survives in the BBC's Heritage Collection and is protoype No.5.