Fashioned after aftermarket brackets by companies like Webco, back in the day.
To suit pretty much all 650 unit machines and likely some pre-unit, judging by the original part number, which is E4711 or 70-4711. Factory versions were made of stamped steel.
These are CNC-machined aluminum BILLET, bead blasted, with polished high areas as shown. Lovely stuff, and should mount with original hardware. They're not that much thicker than stock.