X = W² x C / HB²

Data:

• 10” Rectilinear slide rule, painted surface, printed scales. No scales on rear of slide.
• No box
• Made from some sort of painted hardboard
• Slide has simple angled chamfer running between similarly chamfered stock sides.
• There is a formula on the top arm X = W² x C / HB²
• ‘HB’ is on the slide and the unusual ‘D’ scale is divided in fractions to 21.
• Strange stippling on the otherwise plain reverse of the slide rule.
• Statement of the seller:
  “Unfortunately, I know little about it. It belonged to my dad, who was a naval architect. He was originally from the North East, spent time working in Australia, and finally ended up working in the RNLI in Poole. I imagine it was something he purchased early on in his career, bearing in mind its age, so probably an instrument used during his training in Newcastle.”

Questions:

1. What does this calculate?
   It might have been used to calculate the camber of ship's beams, with HB=half beam. See Fig. 161 of George Simpson, “The naval constructor…”, 1914, pg.371 But why is the D-scale so strange? Andries de Man
   
2. Who might have made it, and when?