The RCA UX201A was introduced in August 1925. The 201A triode dates to 1922 when it was fitted with the then standard UV base. The 201A was the electrode structure of the 1920 201 but fitted with a thoriated tungsten filament in place of the bright emitter pure tungsten of the 201. The UV base has a locating pin that guides the valve into the base before the locking twist into the bayonet base. In 1925 the UX base was introduced by RCA with the UX201A retained the locating pip so that the valve could be used in UV bases. Interestingly the UX201A data-sheet gives the grid and anode connections reversed from the UV201A.
The RCA data-sheet gives the pin connections listed below but counts the pins differently to the now accepted UK definition. If we list today the connects are f, g1, a, f. The UX
The lettering on the base. Radiotron being the RCA trade mark.
Base with locating pip between pins 2 and three.
RCA etched on the dome.
The balloon envelope is 44 mm in diameter, and excluding the UX4 base pins is 99 mm tall.
References: Data-sheet, & 1047. Type UX201A was first introduced in 1922.