Been looking into this type of wrench ... a combination of flare and extra thick open ends. Only thing I don't understand is why Snap-on only have them in 5 sizes ... starts at 3/8" and stops at 5/8"? Their metric are available in much greater range from 8mm to 20mm (13 different sizes).
I have the Mac FC series and have the same issue w/ available sizes...they used to go to 21mm, but now only go to 15mm...might have to lose the Mac's and go to the SO's...