The running board bolts are similar to "elevator bolts", like carriage bolts but with a larger diameter flatter head. You can buy them from Jim Carter LINK. The head is held against the slot by a tab to keep them from turning. You can remove the broken one by tapping it toward the middle of the brace.

Kevin
1951 Chevy 3100 work truck
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Photos
1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.