1957 420U.
I have never been able to find the "proper" square-headed bolts and using hardened regular bolts presents a problem.
The bolt heads sit in the square recesses making them pretty much impossible to get any tool on them.
As a result, the nuts can"t be tightened.
I tried filling the bolt head gaps by welding, but that usually doesn"t solve the problem.
I can get lucky sometimes by tightening the nuts with an impact wrench by simply spinning them fast enough to get some grab.
In the end, whenever I have to remove a back wheel, I have to grind the nuts off and buy new bolts.
In the meantime, the nuts (being a little loose) wobble around and eat the rims causing the need for me to weld washers on the outside of the wheels to provide an actual round hole for the bolts to come through.
Since I"m not the original owner of this machine, I don"t know what the "real" lug bolts should be.
Carriage bolts?
The 420 has some impressive torque...