In the early 1930's a new vacuum tube designed for use in the output stages of battery operated receivers was introduced. This was the type 19 twin triode. Similar to a pair of type 230 or 231 triodes, this tube would produce almost 1 watt of audio in a push pull class B output stage. However since it was really two tubes in one bulb, it could replace a pair of type 230 triodes in any circuit where the 230 was already being used. One of the more popular shortwave receivers at the time was the "Doerle" two tube circuit. This receiver used a pair of type 230 tubes, one as a regenerative detector, the other as a single stage of audio amplification. The #19 dual triode was quickly used in the same circuit to replace the two type '30 triodes to produce a cheap, simple receiver.
To learn more about this radio, I suggest reading the book How to build the Twinplex Regenerative Receiver, by T.J. Lindsay
My Version of this receiver uses a National type "B" vernier dial in place of the round dial that Lindsay used. While this is not what was used in the original receiver, the type "B" dial was available when the Twinplex was designed, and could have been used in home built units of the time.
Check out the pictures of my version of this receiver below.
Update: I've replaced the regeneration pot with a throttle cap mounted on the left and side and moved the filament pot to the right hand side. I also increased the size of the plate load resistor in the detector to 100K. These changes should give smoother control of regeneration and a little more audio recovered from the detector.