Ok all you jokers, it looks like this is in fact a bike tool and the correct answer has already been given above.

After wesmamyke made comments above, I started investigating. As it happens, someone else has also wondered what this is for in the past. The responder gave roughly the same answer as wesmamyke.

So, there you have it. Wesmamyke wins the identification crown and I now know this tool is part of the BBT-8 Park tool which removes and fits the spider lockring on Shimano square taper and octalink cranks.

Now, all I need is the Octalink lockring and I've got a functional, but probably not very regularly used, tool.