What's the Catch?

The latch broke on the side door - inside the door. So every time the dog wanted out or in we had to lock the door just to keep it from blowing open. You see, we still have the original lock mechanism complete with drafty keyhole and with temps well below freezing it just isn't fun to wake up and find the door has been standing open all night.
So I removed the door knobs and pulled the mechanism from the door. When I opened it up, it was obvious that a tension spring had snapped in two. Where do you buy replacement parts for a 100 year old door? Trying to match the make at an antique store would be tedious and time consuming. So I opted to make my own spring :)
It appeared about the diameter of a standard paperclip. After twisting and shaping one it was curious to find that the old spring was also the same length as a paper clip. Perhaps I'm not the first to make such a repair!
While the lock was opened up, I hit it with a shot of WD-40; both for the lubricating effect and to slow down the rust. After replacing all the parts and screwing it back into the door, it works really slick.