I am trawling through the internet as it is a bit grey outside trying to find an answer to the third terminal part of this thread and came across this:
da7c.co.uk/History%20Section … se%203.pdf
I give this 100+% rating - maybe a bit aged in the tooth now the way it is written, but certainly worth downloading if you hobby service the older ignition systems. Explains the theory simply followed up with the testing procedures.
Thanks HCA. That old Lucas technical information especially is really good stuff!!
Regarding the OP’s original question as to the mystery of that third terminal do you think that the coil has been made with 2 voltage taps on the primary side rather than the more usual 1?
By making the coil this way you could in effect have the advantages of a system designed around a ballast resistor without actually having the resistor - i.e one lower voltage primary tap is powered during cranking only and the other powered during normal running? Has some advantages because you aren’t wasting energy by heating up a ballast resistor during normal running.
Youtube clip is again is a simple explanation and how generally most people envisage high voltage is obtained. However an ignition coil has around a turn’s ratio of 80:1. 12v x 80 = 960 volts so clearly, what’s going on, welcome the capacitor.
Unless those trying to understand in detail have a good grasp of magnetic induction, AC theory and pulse techniques the answer will I’m afraid elude them.
Dunno - I still harp back to thinking that if the third terminal is for dual voltage, ballast, earth etc, it it too important for there not to be a mark as to its use. Note how prominently the CB and SW terminals are marked…
If memory serves we used this document or similar back in the early '70s as part of Electricity and Electronics as an O level course/exam, we were guinea pigs or beta testers as we now refer to such unfortunates as this was a completely new subject and, importantly, no previous exam papers to refer to.
I was in the RAF at the time and all the electrical/electronic techy types wanted to do this as it was obviously going to be easy given our intensive, broad-based training and lengthy experience on all types of things electrical (and ‘electronic’).
Twas a humbling day when all but two in a class of 18 failed! The abiding thing was that it (re) taught me was to never assume, start with the basics and ALWAYS use “head before hands”. 
Hi All
sorry a bit late looking at this thread. By and large the way things should work seems to have been sorted but the coil with the 3rd 1/4" fast on(Lucar) terminal is unusual.
getting back to the original post the picture showed a few resistance measurements that I dont fully understand
the SW and CB terminals should measure around 3.5 ohms with a 12V coil but the high voltage output should read to either SW or CB around 4.5k Ohms (4500 Ohms). and what does the 3rd unmarked terminal actually read, and to which if any, of the other terminals.
To be safe I like to ground the body of metal cased coils just for added safety and minimising radio interference.
hope this helps best of luck
Bob
Hi Bob,
Switch to C/B. 4 ohms
Switch to Unknown. 8 ohms
C/B to Unknown. 4.6 ohms
Secondary to all 3 low tension terminals 7k
All connections isolated from the case.
I’ve shown the coil to an aqaintance, now in his seventies who has been in the motor business all his working life. The last thirty five years or so his work was building, servicing, maintaining and competing in rally cars, both here and abroad … He’s not seen a coil like it before.
Here is a response from someone who works in coil production with SMP who really does know his coils:
[i]Hi Hal,
That’s really interesting. I thought I knew my coils – but this is a new one on me! I know we’ve never produced a coil with this 3rd terminal, but I will ask some industry veterans and see if anyone knows what the function is! I will come back to you, I’m eager to find the answer myself now!
Thanks
Pete
Peter Lowery
Senior Product Manager[/i]
Who would have thought that the original simple question would have travelled this far.
I’ll be really interested to see any answer that Peter Lowery can supply. 
Hi Guy’s. I’ll throw this into the mix! I mentioned this coil to a friend who happens to be a boat enthusiast as well as car. He suggested it was for a boat and had seen them before. The third terminal being for the rev counter??
This was my thought at the very start of all this. Not necessarily a boat, But I have a vague memory of some way-out car having a rev-counter pulsed directly from a coil.
We will see
Maybe in the meantime we can make a book on it… 
Evens it is a rev-counter!
It’ll probably have quite a limited appeal even to petrolheads … I’ll not make any retirement plans just yet.
Hi
I think he means a betting book 
I think you’re correct … I’ll go now.