The Unix Time Sharing System (1974) [pdf] (cs.berkeley.edu)

Just guessing, but the label probably became an address on compilation, so one branch of the fork would return in the usual way, the other would jump to the label/address

From the article: "The new processes differ only in that one is considered the parent process: in the parent, control returns directly from the fork, while in the child, control is passed to location label"