Evan,
Most Mark series engines had 'Tappet clearance .005' stamped on the top of the crankcase. If yours has a plate with 6 thou clearance stamped on it, then perhaps the engine has been rebuilt with special cams - in which case, it's anyone's guess as to whether that is measured hot or cold. Also, Mark series engines have steel pushrods, so the clearance hardly changes with temperature - unlike the Plus series and Dragonfly with their alloy pushrods.
Regards,
Eddie.