The Dario RC Special was made in France and the Type does not appear in Thrower's book. The style of tubular envelope and ribbed base shell places this valve in the late 1920s. The RC will stand for resistance/capacitance coupling rather than transformer coupling between stages.
The small diameter cylindrical horizontal anode.
The electrodes are concentric with a single strand filament, sadly broken.
The base has the connection names embossed in the plastic.
The view through the dome. The grid is held on one side with the wire welded to the support.
The thin nickel sheet anode is wrapped into a cylinder.
A closer view of the construction.
The balloon envelope is 38 mm in diameter, and excluding the B4 base pins is 80 mm tall.
References: 1003 & 1043. Type RC Special was first introduced in 1928. See also1928 adverts.