You learn something every day from sites like this.  I'd never heard of a plow bolt before.

I looked these up and it turns out there is an American standard for them, ASME B18.9.  That means something to me, since I made my living from standards.  Of course, there are even more special versions made specially for heavy duty plows (think Caterpillar), but the regular flat head one as used for flatcar decking, and possibly other freight car uses, is shown here --

https://www.aftfasteners.com/plow-bolts-dimensions-technical-information/

For HO modeling purposes, it reduces to a round dot, as David said.  But at least now I know the reason that this is true.

Ron Merrick