Turning my engine over and observing the valve movements it is quite obvious that the painted marks represent TDC (or 4 degrees BTDC) on the compression stroke on the back pot. I suspect whoever built the engine timed it then blobbed the paint on wherever. They may well have used the 4 degree method. As far as I can tell, the slots and cam are aligned properly. I have no idea what the provenance of the cams is. I have been loaned the Elgers collection of half-time pinions to use one to replace the one with the busted tooth, although Ray did say it would probably be OK.