Just J "type"... could be U or unmarked or whatever. Don't need Xs. The main thing is the exhaust port pads. The reason I'm looking for a single head is that I had 308s with the extended pads for the air injection ports, and I needed to do a bit of grinding on mine for exhaust manifold clearance (ported early A manifolds) on the lower pad for the #7 port, and I hit a water jacket (VERY close to the surface, might have been due to excessive core shift, but now I'm gun shy). Trashed a perfectly good 308. Was hoping to find an early version J that still had the shorter pads on the mounting face, so I wouldn't have to grind.