I have a Sommerkamp FT-277B and a President Jackson. I tried one time to build a Frequency counter what shows me the frequency in receiving mode. With the newest firmware you can switch the mode between: Normal, FT-277 Low, FT-277 High and Jackson. The ZF Frequency are in the Firmware, see the Source code.
Here is the Manual: Anleitung
Here is the Partslist for Reichelt: Bauteileliste
Here you can download the Schematics: Universal frequency counter circurit
Here you can download the Firmware: Firmware