They are both legit. From my own watches it looks like Omega started putting the serial on the lugs at the 49xxxxxxx mark, before that most of the ones I have seen do not have the serial number on the lugs, only on the movement. The early 90s was a transition period for Omega and the one that has the serial number on the lug probably came out of the factory line a bit later than the one without. I am basing that observation on pre bond 200m dive watches (I have around 10 of them)
Maybe but this model is at least 5 years younger than the earliest Pre-Bonds and came after that series. I would have expected to see an external serial on a 1993-5 Omega which is the time frame these appeared. My money is on polished off. Either by a clumsy refinisher or if the movement serial is missing too, a grey dealer (or seller of stolen goods!). It was very common back in the day. Personally I could live with the missing lug serial as long as there is a readable serial on the movement. No serial at all is the big red flag and can cause issues down the line.
Yes! Looks good. It’s a mystery why the serial is gone from the outside. I don’t buy that’s it’s old enough to have not had one on a lug but that matters not a jot. It’s all good if it has one inside as far as I’m concerned. Probably a lazy grey dealer.