I would guess a 3dB attenuator, giving a voltage output half the input, so that you can keep a meter in range while you are having a higher voltage output from whatever detector you are using. Standard instrument box, if you undo the 4 screws and look inside there will be a simple voltage divider, probably using 3 resistors, to give the attenuation and provide the required input and output impedances constant, or there will be a small ferrite autotransformer in some cases where they have a well defined operating frequency and impedances.