1/2" Countersunk square coach bolts.

[rustexpert]

Plough bolts with coach type square on head are usually quite short. One place I worked we had a die made up. Had to be spark eroded into a piece of some quite hard tool steel. Just hot rivet the head of a normal bolt of the desired length into the die and grind of the excess. A bit of a big project if you only need a few. I was told and have read that most old fashioned village blacksmiths would have had a selection of such coach bolt head dies mainly in order to supply the village cartwright. Obviously the idea of just making stuff locally as required is just completely ridiculous!