HP8405A Vector Voltmeter

HP8405A Vector Voltmeter (made in 1965?) measures the short term drift of two Agilent E4433 signal generators each set to 10MHz and 0dBm. Both generators have the 1E5 oven option.
The meter shows 1 degree of phase shift in about 8 seconds so the generators are within about 0.00035Hz or 3.5E-11