1950s Omega, no reference on caseback

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I purchased this one here without knowing the reference. I assume it's an FX 6040 based mostly on the lugs, movement, and serial number (14X). But the caseback is not marked with a reference number. Does anyone know why this would be? Could it be a service caseback?
 
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most probably not a US model abut a Swiss case because the known 4 digit ref# of Morris cases is missing. Inscriptions look Swiss to me.