Michael,
Douglas did list a shim about 1.2mm thick to go behind the thrust disc, but if the clutch will free without fitting a shim, leave it as it is. With wear, the clearance will reduce anyway. If you are still concerned about the amount of clearance, check the thickness of the clutch friction plate - it should be 8mm thick. Also, check out the 6 pins that connect the pressure plate to the spring plate - there is a flange on the pins, then the bronze bush, then there should be a thin spacer (about the same thickness as the flange). If these spacers are missing, the thrust clearance will be excessive. Regarding the tufnol thrust disc - it is quite common for the rivets to be flush with the surface. The thrust discs seldom wear to any great extent, but being bigger in diameter than the thrust bearing, any wear becomes obvious.
Regards,
Eddie.