Hi Nigel,
That is an interesting & not very common watch that you have there.
Firstly here is your picture.
This Range is called the "CERMET" range. The models come from around 1982/83 & are fitted with either a 1420 ( Larger Mens version ) or 1380 ( Smaller ladies version ) movement.
Here is an advert from the "Japanese Seamaster Book", giving you a bit more info about the case finish.
The Omega Museum Catalogue also lists some detals about the case finish, which I quote here.
Seamaster Cermet ("Black Tulip"), 1982 -Extra-hard version of the Seamaster with case and bracelet covered by a "Cermet" alloy, namely ceramic (alumina) and metal (titanium carbide) with the exceptional hardness of 2,000 Vickers, making it absolutely scratch-resistant - except by a diamond.
All in All a Seamaster that you don't see very often. I think this one will become collectable in the future.
Thanks for sharing it with us
Steve