I believe it is a single cufflink and I've seen that style of shank many times in antique shops and other places I hunt for things like that, plus I know I at least have a complete pair like yours and maybe more?... I think that style was used from the mid 19th century to the beginning part of the 20th century. I'm going with this one made between 1880 and 1890. On the shank you may even find a makers stamp and that could pinpoint more on the time it was made if you find any history on that maker. There are a couple of cufflink collector sites out there also that you may find more knowledgeable cufflink folks at that could tell you for sure the date on this one. I hunt for the cool looking and collectible cufflinks in antique shops, thrift stores and other places to resale. Some I've found over time have done very well on eBay and made me some serious money. This one cuff was a pretty common style and looks like a brass with a gold wash plate.