Not at all unique particularly with these references, I've seen it before on several different 120s. I'd be much more concerned with the franken movement you have. Caliber 564 is a 24-jewel chronometer movement with quick-set date and was never used in this reference. Someone has put this one together from parts. At the very least the rotor bridge is incorrect, but again, the 564 was not used in these SM 120s.
Now that you know that the movement is incorrect, or at least has some incorrect parts, you should disclose this specifically on your eBay listing. https://www.ebay.com/itm/3235441596...1fc26791670aadc2c103d06fffdf48c&ul_noapp=truePurchases made through these links may earn this site a commission from the eBay Partner Network
Indeed Omega sometimes used double stamped casebacks for this model. Probably they ran out of casebacks and used a caseback from another model (the no date 120). You see this more often and I even owned one. But as @X350 XJR points out. You have a problem with the movement.