The original nut that comes with the binding posts from the OEM are indeed 4-40 x 3/16. On 400's I supply a 4-40 x1/4" keps nut (attached external tooth lockwasher). For 700's, the board mounts to standoffs that screw on to the binding post. I supply a 4-40 screw with attached external lockwasher. As of a few months ago I also supply a standoff on 400's to avoid clearance issues with the W/O backplanes. The standoff is optional if not using backplanes.
If somebody would like 500 or so 4-40 x 3/16" nuts I'd be happy to oblige.