This is a member of Diarrhoea Squad. It is one of many, many conversion I did on the Plastic Plague Marine models. Converting a model to have a special weapon was cheaper than buying a metal model and helped eliminate the repetitive nature of this one pose model. The Mealta Gun was part of an old sprue that was part of the Ork range, this was back when Orks were armed with bolters, mealtas, and what have you.
The model was painted while I lived with Owen so around when 3rd Edition came out. It is pre-tiny line style. It is obviously influenced by Slater who was painting with Owen often at the time. Slater's marine army was brown, though Diarrhoea squad predates Slater's army by several years.