it may well be a "service" part, not an original part, meaning, in this case this is a component made of material that would offer satisfactory wear with both cams.
My '58 GM master parts catalog shows two, it lists all '47-'53, and all '54(excpt P.G. and 261) as one part number (steel) 1865180... and'53-'55 vette, '54 pass w/ P.G., '54-'55 261 w/ 6 volt, and all '55-'58 all 6 cyl 12 volt as cast iron 1929578.
Yeah, they changed to a mutually compatible mystery material.
