You may also notice that the braked one has different axle boxes (the 16A03 side bolt type) too. There are three wheel types valid for use on FR slate wagons, Slaters codes 1618FRC 1'6" 6 Curly Spoke Wagon Wheel, 1618FRD 1'6" 3 Oval Hole Disc Wagon Wheel and 1618FRW 1'6" 1'6" 6 Spoke Web Wagon Wheel, although the curly spoke wheels are far more common than the other two. All three wheel types and both axle box types may be mixed and matched as needed, irrespective of wagon type. Wagons also exist(ed) with mismatched wheel types.
My experience is that if you ask Slaters nicely they will swap wheels and axle boxes for different types when the kits are ordered.
I've built loads of these, and I like 'em gnarly: