Gen or Rep Tudor Hydronaut II ?

[wickfut]

I've found a gen(?) Hydronaut II 20040 for sale in a supposedly reputable online UK watch shop for under £1000.

The advert states it was purchased this year and it also has a windowed watch back. Seeing as I've not heard of the windowed watch backs on these Tudors I'm hesitant.

Any info would be appreciated

Thanks