Hmmm........Secrets list !
 

The comment "sets like these were on the secrets list" the sets are National HRO clone receivers - available to anyone to buy since the late 1930s :confused

https://www.awm.gov.au/collection/C190651

Give the script writers a break, not everyone knew those details.

What I find interesting is the casual disclosure of locations, and to an extent, the capability. A signals-smart opponent would be about to see deeper into the capacity of the Corps than Ma and Pa catching a short clip before the main picture show at the cinema.

Hro
 

Not well known: The AWA company in Sydney made a HRO style receiver for the U.S. Army Signal Corps. during WW2.

http://vk2rh.com/amr101/

More HRO info:
https://www.cryptomuseum.com/df/hro/index.htm