The answer to your question is in the manual you posted.
In the introduction, page 4: "Provision has been made for the user to convert the instrument to operate from an external time base when extreme accuracy is desired."
See also page 11, under the header "E Modifications".
The details are in the schematic, drawn under the power supply.
The internal / external switch is drawn under the power transformer, with A,B and C. Ext Ref output is F, and Ext Ref input is D.
Regards, Gertjan.