I have a fair amount of circular saw blades, and many (not all) of them
have a diamond shape knockout in the center. I have used power tools
since the 1960s and have never seen any saw that fits those blades.
Is this some obsolete design that some blade manufacturers still
include, or is there some brand of saw that still uses that design?
I have at least 10 power saws and they are either Craftsman, Skil,
Black&Decker, and one Milwaukee. Plus a few very old oddball brands,
that I never use.
(I'm mostly talking about the common 7.25" blades and matching saws).