This is a special purpose slide rule for calculating radiation based on exposure levels. Note that each of the disks is graduated with a logarithmic scale.
This particular slide rule was the personal property of Dr. Frederick P. Cranston, Jr., Professor of Physics at Humboldt (1962–85). Dr. Cranston taught nuclear and radiation physics as well as general physics at Humboldt. Prior to coming to Humboldt, Dr. Cranston did weapons research at the Los Alamos Scientific Laboratory (University of California), Los Alamos, New Mexico.
The slide rule is constructed of a 4.5" main disk of plastic with two smaller disks stacked on the front face allowing free rotation of each relative to the others. There is no cursor. Operating instuctions are provided on the reverse, as seen in this photo.
A similar rule, in aluminum, from Eugene Dietzgen is described in the catalog description above, from: Eugene Dietzgen Co. Dietzgen, Seveteenth Edition General Catalog. Chicago, (1955).
Special purpose "specific function" slide rules are discussed in: Peter M. Hopp, Slide Rules: Their History, Models, and Makers, Astragal Press, Mendham (1999).