I think what the OP is referring to is something that looks like a short piece of an old roll-top desk, made from aluminum. The upper end of the awning slides into a groove on that, and the upper end of the "cap" slides into the awning groove on the trailer. As the awning rolls up, the last foot is made up of this cap and it covers the awning material from the sun, and other hazards.
ON EDIT: Can't find a pic of what I'm talking about, but there are other alternatives for a =whole= lot less. Simplest is a vinyl sleeve that simply slips over the awing. Yes, it will be in several pieces, depending on the length of your awning and it =will require a ladder to access, unless you want to roof-walk and lean over the edge to install it. Another neat option is a springy vinyl piece that looks to slip into the awning track and sits =over= the awning. The awning rolls up underneath this piece and it simply rolls up underneath. This is from A&E Awnings.
Lyle