Advice on a 1680 sub

[NickTruswell]

Hi everyone,
I'm looking for some help with a 1978 1680 sub I'm looking at. I'm new to vintage and know it can be a mine field if you don't know the detail. I'm looking for a birth year watch, 1978, ideally a sub or GMT and have seen this one at a dealer in London.

I would very much appreciate your comments as to its condition and originality.

With my basic understanding I can see that the dial and hands have matching patina so probably the same age. The date wheel is silver and not white which is correct but that's about where my expertise finishes.

Pictures below, thanks for your guidance everyone
Nick