The RC210 is a low power triode designed for use as a resistance/capacity coupled amplifier rather than for transformer coupling. The above advert gives the basic operating curve.
The envelope gives the amplification factor as 40 and the anode AC impedance as 67,000 Ω.
The construction uses a glass bead insulator to hold the top components in place.
The tension spring supports a cross bar that joins to the filament that has two vertical strands rather than an inverted V shape.
The balloon envelope is 40 mm in diameter and, excluding the B4 base pins, is 84 mm tall.
References: Advert & Data. Type RC210 was first introduced in 1928. See also1928 adverts.