Flange bolts have a hex or 12 point head.

https://www.imperialsupplies.com/browse/fasteners/cap-screws-bolts/flange-bolts

The bolt part of a crossmember repair kit is actually called a ribbed carriage bolt.

https://www.imperialsupplies.com/item/0009790

When attaching to an aluminum bottom rail a nylon washer is used. For steel against steel it is not necessary. The nut is actually a flange serrated self locking nut. All three pieces are ordered separately at Imperial. We use these bolt a lot because having two crossmember to bottom rivet broken is an out of service criterion. In fact we had a unit put out of service just two weeks ago for having two broken Huck bolts.

https://safer.fmcsa.dot.gov/query.a..._query_string=CRENWELGE%20OIL%20CO%20INC

Had the driver been doing his job on a pre trip inspection, we could have saved a $500 fine and points that raise our insurance.