Aluminium (the inaugural Folding Latch was plastic).
Typically required if original has sheared (structural failure) brought on from stress when transported in the folding position, especially with the earlier plastic versions.
Shearing of the Folding Latch is well explained by Manchild Noir in comment to Bill Dai's Post: "Properly chocking the wheels and strapping the pole and handlebars so that there's no TRAVEL whatsoever will prevent this as well as removing the PIN and the latch will prevent damage as well as soft cushioning to prevent shock and blunt forces."
Image below shows a sheared latch (older plastic type) and the grub screw.