You're missing two things. The "clip" (1968 AIM UPC 5, Sheet C1 Item 14, P/N 3908628) and the "strap" (1968 AIM UPC 12, Sheet B9, Item 4, P/N 3916762). The "strap" is the part that Joe linked.
And if it's an aftermarket mirror, good luck!! Do a search and you'll find a few ways to TRY and tighten it, but most won't last. Anything from peening the housing to superglue!!
Ed