Many available Flat Foot crowns need to be adjusted to fit a 321 movement as they might be a Railmaster (2914) or Seamaster300 (2913) crown. The pertinent stem sizes are
Speedmaster, 321 caliber, 1.0mm
Seamaster300 (2913), 501 caliber, 0.9mm
Railmaster (2914), caliber 284, 1.2mm
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