This case back showed up on a Seamaster 600 from ~64 (for sale at local site).
Haven't seen one looking like this before, made som searches but nothing. All others have a circle around the text, hippocampus and an Ω. No photo of the inside case back available yet, I've asked for one.

Fake or unusual?

 

You have a picture of thé dial ? And the price ?

 

Funny, saw it and wondered if it is a fake, newer seen a back like that, and a caliber 600? would have thought it was a cal. 601

https://www.tradera.com/item/190118/334644272/omega-seamaster-600-crosshair-

 

Got a photo of the inside from the seller. Looks genuine to me, compared it to a 600 i own.

 

I think it is genuine, just rare to find

 

Different but looks all fine.

The dial is referred to as a "technical dial". Several variations and also seen in white.