I need help to identify my omega watch

hi all,
I was hoping someone could identify this watch. I have yet to see one like it.
store short. I think my grandfather bought this watch - possible Portugal, then it was given to my father and then to me.
I have never wore this watch. lately I have been very curious about this watch and even if it is real.
The watch is a 19k gold watch, the crown does not seam to be gold. I am not sure if the bracelet is the original and everyone I can ask is long gone.
I tried to take the back case but the watch came with it so in one piece I had the top assembly of the watch and on the other hand I had the dial and the rest of the back. maybe I did something wrong so I put it back.

this is all to say I have no idea how to separate the back case so that I can see the movement. maybe you guys have an idea.

here are a couple of pictures.
Thanks in advance.

Take it to a watchmaker.

I'm sure people here can help you with a preliminary assessment. I'm surprised by the hallmark on the front of the watch (top left lug). My guess is that it's a third party case made to avoid paying import fees on gold (though I'm not positive the case is 100% real gold. I'm not saying it isn't but think it's strange to see the hallmarks in that place). Maybe someone else can comment on the case.

Thank you all, for your comments.
It is very interesting to me that it took a YouTube video for me to get interested in this old watch. It has been in my possession for over 40 years in a safe. Since it was a gift, I never thought of warring it other than the day it was given to me. I remember that when I put it on, the bracelet was too big, so I put it away.
Now I find it that there does not seem to be others like it, which makes me question the authenticity of this watch.
Still has sentimental value.