it is what they call a 2 piece head. almost all race saws that don't have to be stock appearing are like that and even some stock appearing ones too if they can hide the plug under the cylinder cover. judging by what looks to have been done i bet it was in fact a stock appearing race saw top end. the spark plug will interfere with the cylinder cover so a hole would need to be cut. very cool piece to have for sure but likely would not make a reliable work saw. some guys charge upwards to $700 just to do the mods this cylinder has. i believe the only one offering this kind of work here on AS is Terry Landrum.