I have one of these very inexpensive sets that has all the vet small bits you very seldom use but can be 100% stuck If you do not have them.
So when I needed one of these funny small sizes to do some adjustment to my bicycle brakes I discovered that the handle was broken.
It turned out the hex at the back of the inserts were 4mm.
Quick solution, a 6,, mushroom head screw with 2 nuts locked against one another and a 10mm spanner.
And the quick fix.
Once I have time I will heat up a 6mm tube nut and melt it into the handle and make it a permanent fix.