What do the set screw you have look like .maybe someone substituted an alternate .The set screws I have seen on the handles I have are either a two piece one with a the smaller end then a lock screw that goes in on top to stop it from loosening or the newer one piece either way the end we’re it hits the flat area is smaller than the end of the screw so guessing the screw is about 1/4 and the end flat is about 1/8.
