From this book, assuming it's a cal 13 (not sure of the difference in SO and NN) although I'm not sure how you're identifying it:
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You need a FAMA FLEX 385...
That might be hard so, here are the dimensions
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You've probably got sufficient here if it is a slightly different calibre. At least your watchmaker can check the barrel diameter and height agree before ordering a generic spring. He can also calculate if the length is correct if you assume the strength (thickness) is good because of the space it will occupy in the barrel (basically half of what's available - it uses this when it is relaxed or wound).
Cheers, Chris
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