This is a chanter arrangement for pipers who intend to interchange chanters in the same set of pipes. Normally changing chanters involves exposing the reeds, and this is when they can get damaged. With the split stock arrangement the bag is fitted with a larger stock. Each chanter has its own removable stock which covers the reed and is always left in place unless you need to adjust the reed. This stock then fits into the larger stock in the bag. Thus, when you change chanters the reed is never exposed.
It is a neat design, does not change the outward appearance of the pipes, and makes changing chanters quick and trouble free, as it avoids any possibility of damaging the reeds. It also makes it easier to stow the pipes in their box. I fit split stocks to all my Leicestershire smallpipes and English Great Pipes. I also fit them on my combination Scottish smallpipes. However they can also be fitted to some of my other pipes. Split stocks are also a possibility for some pipe drones.