What am I looking at - spark plug hole #1

[Gilles]

Could be that that the previous owner installed a 'spark plug extension'..? those things where used to keep the spark plug from getting fouled with excess oil (when a particular cylinder was burning an exesive amount of oil), perhaps one of the old gear heads here remembers them..

If I remember correctly, you were supposed to thread them on the spark plug threads, then you bolt the spark plug on top of it .

EDIT: If you install a plug back on this cylinder and measure the distance (depth) from the tip of the plug to the valve cover, and then do it again with another cylinder you can see if the plug on this particular cylinder sticks out further than the other plugs, then you have an extension..