exact item shown, A57 oscillator assembly from HP 8720 and 8719 series network analyzers.  I ran this oscillator for a couple of hours connected to the spectrum analyzer, the two analyzer photos show the oscillator signal at two different resolution bandwidths. the second analyzer photo at 100 kHz span and 1 kHz resolution bandwidth shows you that this device is making a nice narrow peak with very narrow (10 kHz) skirts, and plenty of power - the last photo is an excerpt of the service manual/.  and of course the photo showing the oscillator sitting on a wooden table is to show the setup. This device has serial number 0971