Rob, as you know a carrier has big holes in the top and little ones in the bottom because of which they are very good at being handy bins (I think they are called dumpsters in North America) It (the bolt) doesn't look like it belongs at all unless, as you say, it is from an accessory (or something has been modified)
My darling wife used to put the rubbish bag in mine
(just to show how much she loved it!)