I've just been and checked my old Record 078.
First off, it's an entirely practical design which works well. Some people prefer it to the 778 because you can adjust the depth of cut on the fly if you want to.
In use, the iron rests flat on the body and does not rock on the adjuster. Indeed, the adjuster has some play in it, so even if you tighten the screw right down, it will still rattle.
I measured the depth of the grooves with a depth gauge and they are almost exactly one millimetre deep at the centre, where the adjuster goes.
If yours are shallower than that, you need to make them deeper, swap the iron or very carefully file down the teeth of the adjuster.
Or simpler and reversibly you could stick some paper or thin aluminium onto the iron or the bed, with a cutout for the grooved part, so the iron is raised off the bed a little, and the teeth don't bottom out.