The stainless allen head idea is a good one. Your bumpers are correct and need to use the rectangular tab to attach the end piece. But there are two styles. The one with the tab, which is on my '69 and a set I have that came off of a '72, and the one with the lip on the edge with the chrome piece attached. Here are two pics. The first is from my '69. The second is from a bumper I have with the lip. I have no idea what year that one came off of - maybe a SWB. I have three or four sets. Three have the tab, and one has the lip. The pic with the one that has the lip has the end piece attached and has some dirt on it, but I think you can see it OK. Notice also it has a different type of attachment for the support bracket.
