All the knobs of that style I've seen use hex head setscrews. I suppose it's possible that someone replaced them somewhere along the line with something more arcane. Crud-infested hex heads can appear to be something else--I'd poke at them a bit with a dental pick or something similar, preferably under a magnifying lamp(the middle aged tech's best friend!)and see if you dislodge anything. You might want to apply a bit of some kind of solvent (rubbing alcohol, naptha, WD-40) to loosen accumulated gunk.
