I've completed doing the research in the book "American Military Belt Plates" (by O'Donnell & Campbell), and other books. Because it has four attachment-hooks, your plate is a shoulder-belt ("crossbelt") plate. The AMBP book has photos of the back of every plate shown in the book. Although it shows several "clipped-corners" rectangular plates similar to yours, there is no match-up for one which has the very-unusual shape of attachment-hooks on yours. Those hooks are machine-made, from a piece of thick brass round-bodied wire, with a CONICAL point on one end and the other end is machine-pressed flat. In all my decades as a civil war relics dealer, I've never seen a civil war (or earlier) MILITARY-ISSUE plate with attachment-hooks shaped like yours.
I am particularly concerned by the fact that the pointed tips of the hooks almost touch each other in the middle of your plate's back. That makes fitting the leather belt's end through that very small space difficult. Therefore, the space in the middle between the hooks' tips is much wider on actual Military-issue plates and buckles. (See the photos below.)
Because of your plate's hook-points being VERY close together, and there being no match-up for their exact shape in the various buckle books, I think it may have been made for Military School cadets, or a reproduction made for civil war battle re-enactors. That being said, I do not consider myself a top-level expert on American Military belt plates. So, I suggest you email the photos of your plate to the AMBP book's author, Mike O'Donnell, to get his opinion about your plate. I'll send his email address to you in a T-Net private message.
LOL Pete, sorry I couldn't resist!) 298 with the nice weather, I think most of us are outside instead of inside. Actually he has already looked at your plate, and is doing some research. 
