Surface plate square

Some techniques of genuine toolmaking over looked;
A single threaded fastener and one pin is not doweling, roll pins have no specificity of roundness.
Not much issue with 2 piece base, but relieving underside .020-.040 with a frame or border about .3 wide, reduces surface area to lap and friction in use.
That recess should vent to atmosphere, only requires 1/16th notch or so, preventing a low pressure area and sticking to the plate; or very opposite that skates on film of air right off plate.
I won't endorse grinding on the mill, but many do not have broad facilities....at least drape the ways, and make a simple wheel arbor for a collet, reducing run-out. Clamp the beam between two straight pieces, even wood, to dampen harmonics at each end.
Instead depending on flatness of vise, support ends of such long beam section on convenient matching standards [ie 1-2-3 blocks].
Meanwhile, at that RPM a guard is superfluous.
Grinding alone is decent, but lapping afterwards would be superior.
Reducing face width, maybe not to the so-called 'knife edge', yet will generate a far more precise contact via reduced surface area.
I do like idea bead blasting the sides for glare reduction.

And opening with 'like and subscribe for more great ideas" takes some gall, better reserved for the likes of Abom79, ThisOldTony, Keith Fenner, Myfordboy, Tom Lipton, Keith Rucker, Mr. Pete222, Marc (ThatLazyMachinist),Ca Lem...and dozens more, in no particular order.
And best work ethic, Mike Rowe, hands down.

Surface plate square - video