Haha, we just talked about this 2 weeks ago in a Enicar Group. In this pic it looks higly like it, in the live video it looks more like an UD. On other pics it looks totally different to a guide. Let me find the pics I researched two weeks ago.
The Seiko 6139 Pogue in the photo above looks to be different than what he's wearing in the 'Live in Pompeii' video. Whatever watch Nick is wearing, it definitely seems like it's a EPSA compressor style watch with 2 crowns. Given this, most likely he had several watches...
I guess the TLDR of this is Nick Mason is a watch guy...