The item appears to be a Garter-strap clasp ...used to hold up a stocking or a sock. The strap connected through the rectangular slot at the top.
The key for recognizing this style of garter-clasp is the unusual "hook and retainer-tongue" arrangement. In order to grasp the sock/stocking's fabric without piercing or tearing it, the hook's tip is rounded rather than a sharp-pointed tip ...and the "tongue" just above it kept the fabric from slipping off the rounded hook-point.
As to this item's time-period:
I can't be certain from the photo whether the plating is silver or "nickel-silver." The latter tended to supercede silver-plating of such items in the decades just after the civil war, because nickel-silver is cheaper than real silver, doesn't "wear off" nearly as fast, AND unlike real silver it doesn't tarnish.
If anybody is wondering:
How can an untrained eye tell the difference between plating which is real silver and nickel-silver plating? Compare the difference in the "look" of a silver coin and a freshly-minted nickel 5-cent coin. Real silver is "whiter" in color than nickel.
So... if the plating on this clasp is nickel-silver, my guess is that its time-period is 1870s-1920s. If it is silver-plating, I'd guess Colonial period to 1860s/70s.
By the way ...Nickel-silver plating did exist prior to the civil war ...notably on some Colt pistol brass frame-parts. But it became much more widely used in the 1870s/80s (and after), notably on pocket-watches and other small items which got a lot of handling ...such as nickel-silver plated "silverware."
Regards,
TheCannonballGuy [PCGeorge]
