They were made in the early 1960's and then many have made it through the market via gun shows, flea markets etc in the past 40 some years. They are really proliferating via the online auctions.
The way you can tell them is the angled/flared rivet holes in the specimen. Regular M1859/63 bosses did not have that big, pronounced flare seat for the rivet.
The second clue is the shape of the US and the thick sidewalls of the round field portion. As well, these big heavy creatures are very, very thick from front to back.
The third clue is if you will measure the distance between the 2 rivet holes. The repros will have a rivet span on the centers:
1.5" or 38mm
If they have this rivet span, they will not fit any Civil War era bit, and are reproductions.
The real M1859/63 bosses had a rivet span of:
1.75" or 45mm.
Here are the standard sizing and dimensions for an ORIGINAL M1859/63 bit boss. Note the textured backgroud of the field behind the US as well as the overall design: