The PM6D dates from 1929. The design of the printing matches the PM3 and PM4 boxes. The PM3 carton is date stamped to 1934. The cardboard box is 70mm square and 135mm long.
The top flap is missing but within the top flap is a second plain cover and within that a folded section that provides an inner flat surface across the face of the envelope glass 25 mm below the top of the box. This space provides the top anti-crush zone. Inside this third top protection is a folded inner to hold the valve away from the sides of the box. The valve pins touch the foot of the box.