HP 809B Slotted line with N-connectors for up to ~12 GHz. Unit is missing the 0.500 inch diameter detector plug and the crank/knob for moving the carriage. This does not prevent the instrument from being used. A detector can be made with a 1N23 crystal diode and the carriage can easily be moved by hand. The position can be read off to the 1/10th of a mm (1/250th of an inch!) on the nonie scale. The mechanical precision is excellent!
For best accuracy in readout, the source should be 100% AM modulated with a square wave and a HP 415E SWR Meter used with the detector. Readings to within the 1/100 of a dB are now within reach.