Two receivers for monitoring 500 kc

[Radio KH6O]

In the US Coast Guard, we kept two receivers on 600 meters -- one on 500.000 KHz, and the second one about one KHz off (say, 499.000). The reason for this was that if one receiver was exactly zero-beat with a sending station, the second receiver would pick up the transmission. 

Our receivers (in the 1970s) were Collins 651S-1: 

https://collinsradio.org/cca-collins-historical-archives/the-equipment-of-collins-radio/the-s-word-or-solid-state/651s-1/

Did coastal commercial stations also keep a dual-receiver watch on 500?

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72 / 73 / 3579, 

Jeff KH6O   on 160 acres of mountains and canyons