Radio boulevard has some writeups on the FAA and airline versions of the HRO, take a look at:

https://www.radioblvd.com/NC100.htm

May have seen one or two of the black face FAA/CAA or what ever it was back then receivers at Hamvention before. Speculate that they were only ever used at ground installations, primitive ATC stuff before WW2

Ray F/KA3EKH

From: [email protected] <[email protected]> On Behalf Of Charlie L.
Sent: Friday, February 6, 2026 9:35 AM
To: [email protected]
Subject: [Milsurplus] Not necessarily milsurp, but...

Has anyone seen or heard about a United Airlines  Model ES 276 receiver that is in the National NC100 lineage?  Not a peep, letter, or image of it anywhere.  There is a large hole on the left side that is factory made, maybe intended for the eye tube some models had? No hold in the chassis to show it may have had an eye tube bracket.  The hole on the right side is labeled Range/Weather/Both  which I am thinking in function to the outboard control box for the ARN6 and Bendix RDF with 'ADF1/ Both/ ADF2 /Range'  which is just and audio filter.  but was Cannon plug attached to the audio line out of the RDF's aircraft harness, but not internal. to the RX's

Charlie, W4MEC in NC