This bolt is identifiable by the ridge or surrounding the bolt head, this built-in washer under the head of the Bolt acts to distribute the clamping load over a greater area. A flange bolt is designed to provide the same holding power as a washer. Zinc plating, retards the corrosion rate in a normal atmosphere.
Clipped hardened washers are used where space limitations require one side be clipped. These washers provide a smooth bearing surface and are used under the head of a bolt and / or a nut. The chemical, mechanical, and dimensional requirements for hardened washers are covered under the ASTM specification F436. Clipped hardened washers are through hardened or carburized to add strength, but are physically smaller than a standard F844 flat washer. Whatever you decide to do, DO NOT tighten the two 19mm bolts that hold the mount to the bracket the additional 1/4 turn. Use the new one time tar of 74ft lbs. It will save 60 or 70 bucks finding a new bracket if it strips.
Its diameter is 3/8" and its head is Hex Washer. The material used is Case Hardened Steel and the specification is ANSI B1.1. The thread is coarse and its size is 16. Somewhere there's bound to be a hole in the frame. Weld a piece of gas welding rod or coat hanger to the head of the bolt, then fish it though a hole till it comes through the proper hole. It's possible, but sometimes kind frustrating, getting it from a long ways away. I've even fished one almost 5 feet! Then the nut goes on underneath. Use a grinder after its tight to cut the bolt off flush with the nut so it doesn't hang down. You can even trim it thinner than a regular bolt head if you want for more clearance.