Originally Posted by chipcom
It's a Carradice SQR (Saddle Quick Release) mount. I can quickly put my Pendle saddlebag, which contains all my usual 'stuff', plus room for lunch, jacket, etc. on whatever bike I decide to take out.
Thanks chipcom, I thought it was probably something along those lines. The fact that it's a Carradice is probably why I didn't recognize it. I won't even let myself look at the Carradice stuff right now.