The Museum of the Fur Trade has an iron gang bar mould, but the one I've seen (knowing them, they probably have more than one...) has no markings. Picture a flat iron plate, about 7 or 8 inches wide, about a foot long, with a half dozen grooves down its length for the lead bars, and a handle on one end.
The muzzleloader club I belong to has a bar mould, that casts a 1 pound bar, with the lettering St. Louis Bar Lead cast into the bar. This replicates an original bar of lead, but it isn't a gang mould (although the original surely was), but instead it's a one at a time operation. Still, it's pretty easy to cast up a bunch of bars in a fairly short time once you get the rhythm down. I've got a 5-gallon bucket full of lead bars on end---it's more than I want to lift!
Rod