Being 2.75" wide, it seems a tad big for a pin/brooch, but I think that's what I believe it to be. The 4 sash buckles I have picked up have all had either a narrow slot or a tab. Means either a left piece or right piece. Yours does have the two spots where it could be guessed that the crossbar was tacked onto, but it still lacks either a slot or a tab. I bet those are the spots for the pin and the loop where the end of the pin went into, that's why it's bent the way it is now. Sash buckles were frail, I'll admit, as one brass one had a rolled edge that broke on me and another the bar was broken from the tack (weld).
I guess checking back, there was something almost similar mounting and an open hole, but smaller and oval, that I found recently didn't make it into a thread for me. I shall take a pic of it and show you what I mean and why I feel the item you have is more of a pin/brooch than a sash buckle.
This has a crossbar different than usual, but it still has one and it has the slot. The item that looks like a Chinese fan is the sash buckle:
http://www.americandetectorist.com/...rom-and-a-203-y-o-hotel&highlight=Sash+buckle
This one also has the slot and is more ornate. Pic #8 is after it was cleaned up well. The uncleaned pic shows the normal style bar and it broken from one tack spot.
http://www.americandetectorist.com/...me-quot-what-is-it-quot&highlight=Sash+buckle
This one has the missing bar, but does have the tab and has the two tack spots on the edges. This was the opposite side. The majority have two pieces which are mirror images.
http://www.americandetectorist.com/...16717-A-hunt-with-the-veterans&highlight=sash
And this is a slightly different style altogether. It has the tab, so whatever connecting piece had to be different. No crossbar or spots for it. But there is a tab.
http://www.americandetectorist.com/...-and-a-9-mile-day-in-the-woods&highlight=sash
I'll take a pic of the pin I pulled from a small field which was kinda behind a house built 1850, but I am not certain if it could have belonged to the prop of a nearby cellar dating to mid 1700s. I am still confident the pin is 1800s, but the access looks to be from the corner street, which is closer to the older house. Not pertinent, but not completely irrelevant info. Lol.