A heated XTAL oscillator used to derive 5MHz, 1MHz and 100KHz reference outputs. Short term stability is probably around 1 part in 10^11 with long term stability an order worse. Units like this are often used as the master reference at HF, MF and LF transmitter sites and should be calibrated periodically against a better reference like a Rubidium standard. Without calibration this is just as accurate as any XTAL source as the frequency can usually be adjusted over 1 part in 10^7 and have a fine tune (which if I remember the HP unit is a voltage input) that has a range of 1 part in 10^8 Lee.