There is a system for clamping the stem in a steerer tube where the tube has a pair of cuts in it, one vertical but not extending up quite as far at the top of the tube, the other horizontal-but-angled so that the tube essentially has a pair of ears cut out of it; the clamp squeezes these together and grips the stem.

You still need a top-nut to set and keep the headset adjustment.

Here's a pic of one on a bike (yes I know the stem has a standard wedge, belt-and-braces here):