I've seen some pics of the box, they all said 6d, that's 6 old pence, right?
In the 1925 Melhuish cataloge, there are prices for various stones (page 86 on PDF):
Water of Ayr: -/10, 1/3, 1/6, ie 10d, 15d, 18d, I presume the 18d is for the "full" size 200mm/8in stone.
All below are 200mm/8in...