There are no special requirements under federal law. All firearms made before 1/1/1899 are legal antiques, period.
The burden of proof is on the parties transferring such a firearm to demonstrate that the gun was, in fact, made before 1899. Beware of fakes.
In the case of this particular gun, it appears the barrel is less that 16" long, so be aware of the federal NFA guidelines if it fires conventional metallic cartridge ammunition.