In addition to the usual two 560-series connectors on the rear of the plug-in, the 3T6 has two extra connectors, J24 and P23. J24 is used for programming the 3T6. P23 is a coaxial feed (via a Gremar connector) of the trigger input signal. Both of these connectors mate with corresponding connectors in the 568.
The 3T6 contains a digital delay that controls the time between when the trigger event is recognized and when the sweep ramp is started. The purpose of this delay is the same as any delayed sweep: to zoom in on a region of a waveform that is not immediately after the trigger event. The digital delay in the 3T6 can be controlled by digital rotary switches on the front panel of the 3T6 or remotely, through the 568, via connector J24.
Mod 651C give 100 / 1000 samples/sweep switch. Orginal has 1 / 1000 samples/sweep switch.