Help identifying reference for Seamaster De Ville

Hi everyone,

I am in the process of buying a Omega Seamaster De Ville, but wanted to check if the watch is completely original and what reference number it would be before buying.
The pictures in the listing are quite bad quality and the elderly seller has a hard time to take good pictures on request. Her husband was the first owner of the watch, but she does not know anything about service history/repairs and such, but it looks to be in quite good condition overall and comes with original band, buckle and box.

I will be visiting the seller this Friday, but due to it being a monocoque design I will be unable to check the case for the reference number before buying it...

To me it seems to be a ref 135.020, but on the Omega website this is not listed as being available in solid gold. In fact, it seems all the listed solid gold monocoque Seamaster De Ville's are automatic watches. This makes me hesitant of it being completely original.

Maybe someone can help me out and clarify this? Thanks!

Here is a few pictures I have.

Yes, and I am probably take a shot at this watch either way. I do not have any monocoque watches in my collection and was looking out for one; the gold is always an extra! I am just very curious to know what the reference would be, as I don't seem to find another example like it.

Looks like hallmarks on the top and bottom lugs - which is a good sign.
There is also what looks like a contemporaneous style Omega buckle on the strap - which would be a bonus, especially if also gold.

The case looks pretty good - but check the dial carefully as there are marks and scratches which could be on the crystal or the dial.

Thanks for your replies.
I noted the gold hallmarks as well, but I suppose these hallmarks should not exist on the gold filled cases?
So it is quite possible that this reference (or at least this one in gold) is not listed on the Omega sites?
I did notice some listings for gold capped Omega Seamaster De Ville's with the monocoque case, but these seem to have a stainless steel caseback.

Is that usually the case? If so I did sure overpay for my 18k Constellation.... And I would be overpaying for this watch as well, as the gold value would be 1000 EUR at most. Or is that simply the case because this has less collectable value compared to a Constellation?