Hate to berate you, but I expect others are thinking the same. Anyone attempting to rebuild a carburetor needs extreme attention to details. I don't see how you could maybe lose such a big spring. You should be rebuilding this on a clean table, where nothing can roll away. Put all parts in zip-lock bags and take photos as you go. Your research lacked attention to details. What Dan said was:
"The '64-up units with flat stem have a drive spring but no return spring."
