Hi all, I found additional information on a different Forum, I've attached the relevant information and the link to the source.
"Although gunner's calipers of the continental pattern were known in England through Stone's edition of Bion's book on mathematical instruments (catalogue no. 32), English-made calipers had already settled on a distinctive design. Their two flat blades are hinged so that they slide freely over each other, permitting the steel tips to point either inwards to take the diameters of shot or outwards to measure the bore of a gun. In this example, scales at the hinge give diameters in inches and the corresponding weight of iron shot. The scales for inches of the bore and for the pounder rating of the gun are engraved along the length of one of the limbs. The remaining space on the limbs is occupied with tables of specific weights and quantities of powder for iron and brass guns, as well as a sectoral scale of lines and a series of diagrams representing proportional relationships between different planes and solids."
http://www.mhs.ox.ac.uk/geometry/cat10.htm
Greg