Hello NLRS land Dan, K0TI, has a plug-in for an HP freq counter for sale. It is a HP 5255A Freq Convertor 3-12.4 GHz that plugs into an HP 5354 freq counter. (probably other models as well). It is very clean, looks like it was never used, but has not been tested. If you have an interest, drop Dan a note at dkarg at ekarg.com . 73, Jon W0ZQ