The metal thing with the hose to the carb is the vacuum advance. It's designed to advance the ignition a bit more when you go off the throttle. It's an "economy device" because extra advance essentially means more mechanical energy out of the engine ...
Not quite right Anders I'm afraid. The way the vacuum advance works is this: The mixture is fired just before top dead centre because it takes a small amount of time to burn and produce the power and if it is fired at just the right point then the power is produced just after the piston has gone past top dead centre, and therefore it pushes the piston down rapidly to give that power to the crankshaft. If the mixture is fired too early it tries to stop the piston reaching top dead centre and thus stop the engine; and if it is fired too late the burst of energy is wasted as the piston has already gone down.
Now this is the critical point, a weaker mixture takes longer to burn, so you need to fire it earlier to produce the power at the same correct point just after top dead centre. So as you lift the throttle slightly to stop accelerating and start cruising the mixture gets weaker and you need to advance the ignition point. Since the vacuum increases as you lift the throttle, this is used to pull the timing into advance to fire the mixture earlier. The combination of the smaller throttle opening, higher vacuum, and extra timing advance add up to a more economical cruise condition.
Very high performance engines do not use this system since the timing advance on the overrun can over advance the ignition causing engine damage, so cars like the Mini-Cooper S and Lotus twin cam for instance had distributors without any vacuum advance mechanisms. A more economical cruise condition was not really a requirement with these cars! :-)
David, as Anders pointed out, you must not rotate the distributor otherwise you will alter the ignition, and that is a fixed requirement. There is a way to alter it, but it is more involved. You remove the distributor AND the drive gear, and rotate the drive gear meshing one or two teeth, then refit the distributor and re-time the engine with a strobe light.
The distributor is held by a 'Y' shaped clamp with a bolt which is very difficult to get at unfortunately, so for the moment you should leave it alone. I can do the modification for you some time if you wish.
Roy