Sorry, just seeing this now. It's assuredly a sliderule, which would have been a common tool for engineers and mathematicians of his era to have had. They became less common with the advent and proliferation of calculators.

For those interested in looking at a system in English but with a slightly different form, but ostensibly similar, try W. Ross Ashby's digitized note collection: http://www.rossashby.info/

Perhaps not coincidentally, Ashby was a research colleague of Luhmann's.