Omega f300hz Seamaster with a sigma dial?

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Hey everyone! I am thinking of picking up a f300hz Seamaster and found one with sigma dial markings at the bottom of the dial.

I have never seen a sigma dial on a f300 and was wondering...

1.) Is it legit? Did they even make these?
2.) Are they common and I have just been under a rock?
3.) Do they command a premium as far as pricing?

Thank you!
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