Collins,
in the early 50's started manufacturing OEM after market filters
for both the early Collins A-Line and other manufacturer's radios
such as the National HRO-50/60 radios. These are a nice addition
improving the radio's selectivity by adding mechanical filters
in to the radio IF chain.
800Hz for the HRO-50T. Note the plate
cap pigtail
3.1KHz for the HRO-50T. Note the plate
cap pigtail
