Maybe this discussion would be more beneficial with some pictures.
Start with the orientation of the distributor drive shaft in the block.
Page B32 in the XK120 Service Manual has this paragraph.
This means that when the #6 front cylinder is ready to fire, the distributor drive should have the fat part close to the block, slot away from the block, and look about like this:
There are 20 teeth on the brass drive gear, so if you are off, it will be 18 degrees or a multiple of 18 degrees off.
The offset driving dog is offset about 2/32" or 1.5mm
It should only go in one way. If you put it in 180 degrees off, it may sort of catch in the slot, but will probably not sit down in all the way.
Now the rotor should point forward.
If it points backward, your points cam shaft is in 180 degrees off.
It hooks into the two centrifugal weights. Disassemble the top part of the distributor to change it.
Now set the wires in the correct firing order.
