HI Chill...
If you don't mind, could you post your SN? Not the full SN, just leave the last two digits blank.
I suspect .. but don't know for sure, that the first step you come to as you start at the tail of the bolt, and move to the right, is the place they're referring to. Maybe what they're trying to describe is the older 'U' shape, actually digs deeper into the metal .. in other words, there's a trough between the tail, and the first step (where the material gets thicker). Alternatively, maybe the "L" indicates that it simply steps up to the thicker material, without first getting deeper. So, the "L" shape would be a 90 degree angle at the meeting point, and the "U" shape would be a shallow groove cut into the material, at the step.
I'm just as clueless about this as you are, but yours is new info so I'm speculating .... I'l pull mine out tomorrow and check it out. Thanks for taking the time to post the pics and the info.