I came across a review on another site (All4shooters.com) that discussed the design differences between the A1 and the S1. Item one is that the S1 trigger pack well, is smaller than the A1 well, and is made of metal so it can't physically accept, or be modified to accept the A1 trigger pack. Secondly the A1 and S1 uppers and lowers are not dimensionally compatible with each other. So they can't be swapped out with each other. Maybe there is some hope we will see these here.