Strato-charged or X-torq in Husky speak is another butterfly above the carb to send air in to clear exhaust, rather than a fuel charge. Less emissions. Started in ~2006. The old square block carb has an added plate with the extra butterfly - real tall carb results.
Pre-strato Poulans are from ~1990-2005.
Look at ebay pics for difference...search Poulan 4018 carb, then Poulan 2150 carb.