All the pictures I can find of a Gibson J-40 look absolutely bog standard.

Pegbridge (remove peg after slackening string to detach string and insert ball end of new string) and upright machine heads (not slotted head) - so thread string through correct hole after removing old string, allow one fret of slack, and wind on so that the string goes the side of the post AWAY from the edge of the headstock.

This should give you three turns (roughly) around the post.

Genuine Gibson J-40 in a flea market - some chaps have all the luck.