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All the period catalogue photos show plugs and plug leads to the rear of the cylinder head. If you were lucky enough to find a twin spark magneto you could have four plugs and leads! Klaus Kretzschmar in Bonn has done just that after I managed to find a twin-spark Bosch magneto as fitted to the 1914 TT bikes.
Some of these early 1920’s barrels have the plug holes like Erik’s but others have them splayed out at an angle. I don’t know what that was supposed to achieve. The earlier Standard Model barrels were of course different again, with one plug hole on the top, and one on the rear face of the cylinder. This is probably a throwback to veteran days when most car and bike engines had primer taps on top of the cylinders, for squirting in some petrol for the first start of the day, mainly to thin down the very sticky oils of the period, which made engines stiff and difficult to spin over quickly enough for a cold start. It also helped give a rich start mixture that was hard to achieve with the more primitive carburetters of the era. The earlier veteran Scotts have a holder on the frame to carry the petrol/oil squirt syringe, but it is a while since I saw one in-situ.