This photo shows the hollow stock used on this set.
The three drone reeds sit together in a chamber within the stock. This results in an interaction between the three reeds, possibly helping the drone sound, as well as causing the wall of the stock itself becoming a resonating surface which transmits the buzzing of the reeds.
The tenor and baritone regulator reeds sit in brass tubes running the length of the stock, as they react very badly to being the same space as the drone reeds. The bass regulator also gets an air supply isolated from the drone chamber and is carried by a small brass tube which open at the back end of the stock.