I just put a new historical document out on our website, r-390a.net,
courtesy of Les Locklear. It's about the Presidential Communications
Railroad Car used from 1942 through 1961.

 Here's a direct link to it:
The Presidential Communications Railroad Car / “ (coldwar-c4i.net)

<https://coldwar-c4i.net/crate/the_presidential_communications_railroad_car_crate.pdf>

In the picture on page 3 from 1956, notice the radio control console
contained 2 SP-600's and 2 R-390A's. See the picture on page 8 and notice
in 1961 that there are 4 R-390A's. Enjoy. Thanks Les.