Now I am a bit surprised.
People haven’t heard of inbuilt ballast resistors. This may be the wrong way to describe the difference between coils that are built to have an external resistor and ones that don’t due to higher internal resistance if so I am sorry but every one I have ever spoken to refers to them as internal or external.
To make sure I am not confusing people I will explain what I was talking about. This only refers to early English cars such as S1 Land Rovers and Minis etc.
As an example of a car that uses an external resistor let use a Holden Red motor, the resistance is built into the wiring, so that when stating the full battery voltage is applied to the coil and when you release the key back to on the coil is supplied thought the external resister and as someone stated probably around 9V. This improves starting.
If you run a coil that needs an external ballast resistor on a S1 the coil will run very hot thus its life is shortened and they can intermittently break down.
I used the Bosch GT40 as an example as it is the only one still available that I know of and is commonly stocked, as a reference it even states Series 1 Land Rover on the packet. Remember this coil also comes in two other formats GT40R and GT40RT that do not suit early English cars. You may still be able to get Lucas but I don’t know where and the GT40 packs a good punch. You need to paint it to get rid of the orange/red colour not very English.
A quick Google of automotive coils will confirm what I stated or talk to your friendly auto elec, if he is over 40. Feel your S1 coil after a 20 K drive if it takes your skin off it is the wrong one.
Sorry if my first post confused people but this is a common mistake and I have burnt my hand on more than one series Land Rover due to it having the wrong coil