For those confused about where spark plug wires connect to the distributor cap, here's a couple of pictures.

Photo on left is the standard Lucas vacuum advance distributor with side entry distributor cap. Photo on right is my Mallory Dual Points distributor with stop entry distributor cap (and no vacuum advance parts). Lucas distributors with top entry caps have the same connector positions.

When the distributor drive gear is installed with correct orientation (see IG103), and #1 piston is at Top Dead Center on compression (both valves closed), the distributor rotor will be pointing roughly in the direction of #1 spark plug (about 1:30 o'clock direction). Firing order starts there and proceeds anti-clockwise around the distributor cap, 1-3-4-2.

Notice orientation of the hand held Lucas distributor on the left with vacuum unit pointing straight up, and the cap retaining clip slightly to right of being straight up. With everything properly aligned, that clip should be very near to direct alignment with the open slot of the base clamp plate.

Then look at orientation of the Mallory distributor on right, with incorrect orientation about 10 degrees to the left of straight up. With no vacuum advance unit to cause interference issues, this didn't bother me for 30 years, until the time the car was dead on the side of the road, and I had to swap out distributors in the dark with a steady rain.

Then the Lucas vacuum unit would sit either 20 degrees to the left where it hit the oil pressure signal hose, or 20 degrees to the right where it hit the oil level dip stick, depending on which tooth of the drive gear was engaged. I first tried to drive out the cross pin to rotate the dizzy drive dog 180 degrees (the quickest fix under the circumstances), but it could not be reassembled that way because the cross hole was not on center of the shaft (drilled in assembly I suppose). Then I had to put the drive dog back the way it really belonged, and also reorient the drive gear in the engine before the Lucas distributor could be installed the right way with vacuum unit pointing straight up.

If you have to do that just once in the dark and the rain, you would never again EVER tolerate the drive gear being installed wrong. Please fix it before it bites you in the ass.

For proper drive gear orientation, see IG-103.