A much more enlightening bit just arrived in the mail:
The forks are the pivot point for a hammer connected to a treadle -- the final welding blows were foot powered. This was referred to as a Tommy Hammer shaping and scarfing were done with a hand hammer. Joining the link to the chain and welding was done with the treadle striking the welding blow on the finger like projection on the left side of the anvil.
I have used one and it's really a hoot and very efficient.
Iron Orchid Forge
Jim also said he'd try to run down some photos that show this setup in use. And he did. And it took me eight years (!?) to get around to putting them here? How the time fly. But here they are .