W4RT OBF-Y-500HZ One-Plug Filter TM CW Filter for the FT-817 FT-857 &
[opf-ft-500HZ]

One-Plug Filter TM CW 500 HZ Filter for the FT-817 FT-857 & FT-897 

This excellent filter contains a 7-pole Collins Mechanical Filter essentially identical in performance to that Yaesu uses in the YF-122C optional 500-Hz CW filter. 

Connectors like those on the YF122C allow you to install and remove this filter just as you do with the Yaesu filter. Filter unit contains the coding for the FT-857 or FT-897 to recognize it as a 500 Hz CW filter.