SCdirtdigger's looks smaller than the "controversial" pronged plates, so I don't know if it falls into this category. Even with the penny in the picture, I can't tell exactly how big it is.
Anyway, I'm going to weigh in on these objects in general, since many, including myself, have found and posted them on this site. My friends and I found two that were were oval and one rectangular with clipped corners. My feeling is that they are shoulder belt plates, even though they are not functioning buckles. Here is some of my reasoning:
They are often found at sites with Revolutionary War and War of 1812 artifacts (ours were).
They are the same shapes and sizes as the functioning buckle versions.
There are field-cast buttons, where buttons were "home made" presumably to make a uniform look right. These plates may have been crudely made, just to make uniforms look right.
The shoulder belts could be hung over the shoulder without unbuckling, so a fixed plate could work.
Brass nameplates were used on trunks in the early 19th century, but all of the ones that I have seen are through-fastened, and I haven't seen any that are the size and shape of shoulder belt plates.
There are a lot of antique trunks around, so with all of these objects being dug, there should be some existing antique trunks with shoulder-belt-looking tags. As smokeythecat said the trunk plates tend to be smaller.
They might have been used on saddle bag flaps or saddles in some way, but I have not seen any paintings/illustrations, showing large, plain plates on either, so I don't know why that explanation would be favored over the shoulder belt theory.
Obviously, I've thought too much about this.
