I worked selling jewelry for a couple of years while in college and occasionally ordered a shank like that. Gold wears out, so latches that hold the ring shank closed must be repaired now and again. If it came off like that, its time to take it to a good, manufacturing jeweler to reinforce and renew that latch. Think of it, nearly half a century of wear on a tiny latch? For your own comfort, have the jeweler mark the locations of inclusions in the diamond so it doesn't get switched out.
Odd thing, did you know you can have your ashes converted into a diamond? It keeps a sparkle in the eye long after you're dust.