Lost knowledge

Back in the day, way before Clive Sinclair invented the pocket calculator, slide rules were all the rage for performing complex calculations. By today's uber-automated, touch-of-a-button standards, they were fabulously arcane devices, etched with numbers and scales of various sorts that you could move ('slide') and align to find your answers (usually based on completing logarithmic additions and subtractions).

This is a particular variant from the world of printing, the Rothmill Paper Trade Rule, designed so it tells us by one John Black of Markinch (but US patent applied for...). I think it dates from the early years of the last century. Literally every surface and edge of the device carries a scale or measurement and it wouldn't surprise me if there was no-one left who knows how to work it or even what it was used for. My inner geek is crying out to know - suggestions on a postcard, please...