National got the contract for the ground-based airport receivers. and his West Coast design team were involved in some of the electronic engineering work of the new receiver that was designated RHM.
Actually, the Comet Pro was only $155 (and it had a built-in power supply) but it didn't have an RF stage and required an external pre-selector for image-free reception above 10mc.
The Comet Pro came out in 1931 and, from 1932 up to about early 1934, only National and Hammarlund were offering commercially-built, shortwave superhets.
The RIO didn't use plug-in coils but had two switched tuning ranges that covered frequencies below 500kc.
I have owned the RHM receiver shown above since 1990.
In addition to the RHM and AGS receivers, National also produced the RHP and RHQ receivers that were very similar to the RHM within the circuitry but ganged the three coils together behind a small panel that created a plug-in "coil set" for each tuning range covered.