Nathan,
Vintage lens have their own measurement for Shutter and Barrel sizes. They are not standardized from one brand to another. Modern shutters are standardized to Copal sizes. I do not know when the Copal standardization came about, probably the 1960's or 1970's. Best bet when buying used lens boards is by hole diameter not by shutter number size. If a seller cannot provide the hole diameter in millimeters skip it.

Now, as previously mentioned, making your own is not that hard.
A 4x5 Pacemaker lens board is: Left - right = 3.710 inch; Top - bottom = 3.648 inch.
1/4 inch baltic birch plywood available at craft stores works well for lens boards. Start by cutting a blank then round the corners so that the blank just sits into the front standard opening. From the back mark the bellows opening. Remove the material outside of the bellows opening mark leaving a 1/16 - 3/32 thick lip between the bellows opening and outer edge of the board so that the board just sits into the front standard and the board locks just slide over the board. Drill the mount hole countersinking it if needed for the lens to fit. Paint the back side flat black, finish the front as you like. The board fitting into the bellows opening negates the need for the lip on the outer edge for the light trap.

A Dremel in a router attachment with a straight cut bit works well for reducing the outer edge as does a table saw.
A brake cylinder hone in a drill should work for enlarging a metal board hole also.
Black acrylic works well also but is harder to work with. Sand the gloss off the back side. Black ABS sheet is difficult to work with and is slightly flexible.

The factory aluminum boards are .049 inch thick.