In The Wrath of Khan, Khan wears a broken Starfleet insignia around his neck:
At some point, I convinced myself that he snatched it from one of the Reliant crew members, given that its style was contemporary for the time, and given that
"Admiral Kirk never bothered to check on our progress"
suggesting that they hadn't had much contact with Starfleet since their relocation to Ceti Alpha V. For comparison, here is what the insignia looked like at the time Khan was exiled — basically a patch embroidered into the garment:
However, I decided to check when we first see Khan sporting the insignia in Wrath, and he is in fact already wearing it on Ceti Alpha V:
The question is:
Who did Khan get this insignia from?