My understanding on the ranchero is that they are encapsulated in a faraday cage.
The dial is iron, there should be an iron movement ring and then there should be an iron dust cap under the case back.
This is the civilian version of the RAF 1953 casing system.
I will check my Omega book and see if the case reference is correct.
Update.
So not all rancheros had the iron dust cap for antimagnetic protection
Cheers
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