[ARC5] What is a "radio range filter"?

[J Mcvey]

I was reading the audio operation description on the SCR-274 manual pdf page 28, document page 16 . There, near the end of the first paragraph it say that for single place airplane, the "pilots headphone is plugged into Tel-A though a radio range filter, if used". No nomenclature given!  What is it and what what it's purpose?
I have heard that here was some kind of passive tone filter they used to use to narrow the audio response, but I have never seen such a device. Anybody know about it?I was wondering if it might serve as a cw filter.
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