In August 2005 a different type anti-run-on valve appeared on eBay auction. See picture. This is NOT the same as the factory MGB part, and does not work the same way.

This one works by venting a large amount of raw air into the intake manifold. This does not stop all fuel from entering via the carburetor(s), but it will make the fuel mixture go extremely lean, and that will "probably" do the trick (still not entirely guaranteed if it's running very rich). This requires a very large hose connection to the intake manifold, similar to the hose size used for a brake booster.

You may notice this ARO valve has a large intake air filter for venting air into the intake manifold. You need to ask if this is a normally open or normally closed solenoid valve, as this makes a BIG difference in how it has to be wired.

If it is a normally open solenoid valve, venting in the absence of electrical power, it would need to have power applied constantly to allow the engine to run. This could be very simply wired to the ignition switch, or to the input to the ignition coil (before any ballast resistor), or to a switched fuse. The solenoid would then be constantly energized any time the ignition is switched on.

If it is a normally closed solenoid valve, the wiring gets more complex. To make it work on the MGA you would need a normally closed relay contact to supply power ONLY when the ignition is switched off, and a normally open oil pressure switch to supply electrical ground ONLY when the engine is turning with oil pressure. Then when you switch off the anti-run-on valve will vent the intake manifold, and when oil pressure drops with engine stop, the power will be removed from the anti-run-on valve.

Problem here is that this ARO valve seems to have only one wire and is probably grounded through the mounting bracket. In that case you would need another relay to work in concert with the oil pressure switch to interrupt input power when there is no oil pressure (after the engine stops tuning).