Could be a storage box for his lures or it could be a set of scales to weigh the fish before releasing it. I have both and they do look like that. Unfortunately as I didn't take the picture I can't be more helpful.
If you wading you have to carry everything you may need with you as it is a pain in the arse to have to keep returning to the riverbank for piece of equipment.
That is of course the serious answer ~ if on the other hand you are just being frivolous then piss off.