It might be a form of Library Strip.
The one I've seen is metal with multiple screw fixings to the side of the bookcase. It can be recessed into a groove or surface-mounted. It has slots that little brackets hook into. Books are very heavy. The idea is that you can easily move or add shelves to suit the size of books.
the version in your pic, the metal would presumably protect the holes in the wood from wearing loose or giving way and letting the pegs bend downward.
I would have expected a spade-shaped peg though.
I have some where the metal bracket, similat to the toolstation one, has another little peg projecting upwards so it engages in a shallow hole you drill in the underside of the shelf. This prevents the shelf being moved and coming off the bracket. In my case I got them when shelves of books put enough sideways pressure on the sides of the cabinet to bow them out, and the shelves came off the little plastic brackets and fell down.
edit
Found them!
They do take longer to fit, as you have to measure and drill the shelves. I marked mine with Engineers Blue on the pegs but it would be quicker to make up a marking template, or be good at measuring.
mine screw into the cabinet for greater security
I usually buy Hafele but I don't recall what make they are.