This rotor is almost certainly an '81 up 930 front
Here is an unmounted one 32x304 note the unusual hole pattern
The only 28mm rears were
930 @309x28
965 @299x28
993RS/tt @322x32
the 28mm fronts were
964 @ 298x28
944t, 944S2,968, 968CS @299x28 these are 17mm from the rotor face to the mounting face for <'86 and 46mm >'86