Have you taken that off your bike? Either way, you Sir, are a gentleman and a scholar. That looks like an ordinary circlip to me.
It was on a frame without a chain (one of 2012's builds that is now a 2013 build!) so no drama at all - glad it helped it you out.
I've had so much great help & advice on this forum that if once in a blue moon I can help others than that's what it's all about isn't it.
yes, indeed, bog standard clip - one small word of advice if I can be so bold - don't overtension the spring into the internal bracket else you end up with a free paper weight - http://www.retrobike.co.uk/forum/viewtopic.php?t=230923
luckily the 840 mech only has two holes for the spring but just be a bit careful