Originals are made by Cliff, their
S4 series. Rean (Neutrik), CHK and others make clones (though I've not seen a Switchcraft clone).
Note that the originals have a 3/8" BSF thread pitch (20 TPI), while some others have a different thread pitch, 3/8" UNF (24TPI) if I remember correctly, and are not interchangeable.
EDIT: and others still, (such as
Rean) have a M9x0.75 pitch (roughly 34TPI).