"This was found with old tools. Doesn’t seem to attach to anything. It can lock in place and the middle bit turns round, unravelling two straps of fabric. I’m stumped!"
A: "It’s for carrying books."
this is all lifted from a reddit thread.
https://www.reddit.com/r/whatisthisthing/
https://www.reddit.com/r/Whatisthis/
Most humor sites repost reddit stuff.