I purchased a set of Magnecor spark plug leads for my 560 SEC some while back now. The original plug leads had a resistor built into the end caps on the plug end, but the leads i fitted do not have them.
I was having a few bits fixed on my SEC by a very good friend, Talbir (who some of you...
Easily the funniest thing I've seen on eBay, the Power Resistor of Justice answers your every tuning requirement. Check the Q & A section for the best bits...