The B170-V is a 170 Ω attenuator for the 517. Switches select from 0 to 64 dB of attenuation.
John Addis wrote:
The inductors compensate for the switch stray capacitances. When the Ls and the stray Cs are connected like that, they constitute a lumped element delay line. The intent is not to make a delay line, but compensate out all the stray Cs by making a lumped delay line out of it. That extends the bandwidth and makes the impedance 170 ohms (up to 1 GHz or so) rather than a bunch of Cs sitting on the connection from input to output.