I had a Williams superslim 10-mm spanner which is open ended and square that the other not sure if searching eBay will come up with one they usually round off and you'll end up taking the brake shoes off heating up the adjuster and putting a pair of curved jaw mole grips on the inside to get them to move at that point providing they're not rounded off you can use a conventional 10 mm open ended spanner
I am using Beta 1471CH for many years with excellent results. Can reach front adjuster on rear drums too, and as a bonus it looses stubborn gearbox drain plugs very efficiently. Check also Facom T.263A. Both of them have been out of production for some years, I think, but wit some searching it may be possible to locate one.