I have only repaired a few of these clocks but I would imagine your guess is correct and its been put together from parts. If you can get the spring to roughly the correct length and then try and run the clock you should soon be able to tell if the spring thickness is correct. If I remember correctly, if the spring is too thick the clock will run a little on the slow side, and vice versa if the spring is too thin. The lever position which connects to the anchor is one of the most critical things, just check you dont have “flutter” when the clock is running. Good luck with this one and please let us know how it goes.
Paul 
