Thanks for the help.It can be difficult with these earlier movements which have multiple (unstamped) caliber numbers or names to figure out parts info. You may have to measure the height and thickness of the current spring (assuming it itself is correct) to narrow down the search. I can't quite figure out from an internet search if the caliber inside is the 54A, Cronos, Crown, Crown Cronos, 810... like I said, it can be difficult if you don't know what caliber you are dealing with. Maybe have your watchmaker identify the caliber and measure the existing spring? They would also be in the best position to find a replacement spring as well.