I'm going to have to look more closely at it, maybe get a photo or two. I'm familiar with the grooves you mentioned, but I don't recall seeing those on these headrests (which is not to say they aren't there). Aren't the ends of the headrest shafts tapered to get it past that spring-loaded detent? I asked the previous owner, and he couldn't shed any light on it; he said he never tried to adjust them. For the record, they won't come out either. They're absolutely stuck in their current position.