I decided not to procrastinate any longer, and I have a Bosch Silver Plus due for delivery today (same as the one that's been in the car since May 2014). There is a spur on the +ve lead, with a rectangular black-cased item that is a real "wotzit" for me. Any ideas what it's for ?

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Peter
2009 3-litre Roadster "Ivor", royal ivory / green