Just to add to the discussion. Here's the best closeup I was able to get. It shows how very slightly the Hex protrudes above the adjacent surface. I do have an assortment of small wrenches nothing works so far mostly because of how little edge there is available to put force on. But as "Big Train" suggested a 3.0 hex driver may grab this. I don't have one unfortunately.
Sometimes the brass mfr’s put a nut driver(s) in the box ......might you have some other brands in your stuff? .....might be time to invest in a descent set of micro metric nut drivers.....you’ll need a thin wall for that bolt for sure......Pat