Thanks for your comment...
The 7B53A external amplifier bandwidth is only specified for use in a horizontal compartment. Only 2 MHz, which is also the maximum for a 7623A.Indeed you're right, I was having the figure of 100MHz in mind, which is the freq. response of the 7B53A's trigger system.
I was being fooled by the fact that the external triggering enters exactly through the same path of the external "AMPL" input signal. However, after the front end attenuators and FET buffer, the signal splits and takes different paths, to the trigger system, and to the Horizontal pre-amplifier output...
The path from the EXT input to the trigger system may very well be 100MHz, but it never comes out of from the 7B53A. It is only used internally to trigger the TB (so we never really look at it's waveform or step response... Only the trigger system "sees" it.