Up for auction is a field artillery plotting instrument made by Union Instrument Corp., Plainfield N.J. This metal protractor is called a Range Deflection Protractor (RDP) or fan protractor and was made for the US Military. The serial number is 8483-1-2 and the scale is 1:25,000. I did find some info online that I am including below.

The protractor is in good condition. It does have markings in pencil on it from use. I could not find a date of production.

"The RDP is used to measure angles in mils and distances in meters. Range and deflection are measured from a firing unit to a target. Direction and distance are measured from an observer to a target.

The left edge of the instrument is the arm and is used to measure range or distance. It is graduated in 50-meter increments and labeled every 500 meters on a scale of 1:25,000. Ranges and distances are visually interpolated to an accuracy of 10 meters. The arm can be labeled to represent charge or range spans and other pertinent data to aid the FDO.

The 1,000-mil arc of the RDP is graduated in 5-mil increments. The 50-mil increments are indicated by longer graduations and are permanently numbered. The are is visually interpolated to an accuracy of 1 mil.

The vertex, the slotted portion of the RDP, is placed against a plotting pinto properly
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