I think the distance from the top of the cross slide to centre height is about 3/4" so the standard arrangement of a simple clamp could hold monster size tools.
Clearly it was never anticipated to use 3/4" tools clamped directly to the cross slide, but to use holders or packing with smaller tools.
Most people use an aftermarket toolpost to takes 8-10mm (5/16" – 3/8"
tools.
Neil