Here's a replacement key for your Hermle 340 or 341 series clock movement. It will also fit SOME Seth Thomas, Gilbert, Sessions, Ingraham, Waterbury and many other antique clocks. This key is for 4.25mm shafts. This key fits many antique American clocks.
To confirm the key is correct size (no need if for Hermle 340 or 341) measure the square winding shaft. The square opening of this key is 4.25 millimeters.
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