Said switch is on the front panel of the 7104 - turn the readout pot fully clockwise into the detent at the end (PULSED position). To quote page 2-10 of the manual:
The READOUT control determines the operating mode of
the Readout System. With the READOUT control set to
free run (out of OFF or PULSED detent positions) the
Readout System operates continuously, interrupting the
crt display at random (for about 20 microseconds) in
order to write each character on the crt. With the
READOUT control set to the PULSED position, the
Readout System operates in a triggered mode; one
complete frame (up to eight words) of readout is
displayed. The + GATE or EXT switch determines
whether readout is displayed at the end of the + GATE
or when an external signal is applied to the rear-panel
GRATICULE/READOUT SINGLE SHOT input. The+ GATE
switch selects whether A gate or B gate triggers the
readout.
HtH
David
David C. Partridge