It appears to be one of the Great War Miniatures standard Lewis gun poses, used in both the Lewis Gunners and Brit Infantry in Gas Masks packs. But he's been given a new head. Though with the other two uses of this pose one has a leather jerkin, and the other has a sheepskin one, whereas this fellow seems to have no jerkin at all. I'm wondering if it was a one-off made by Great War from that same pose that was never put into general production? I dunno, more likely that it is just a conversion I guess.