Four-slide slide rule

Data:

• 6.7 cm × 55.5 cm × 8 cm
• Materials: wood, metal, tape, paper
• Date Made:1939-1987

Question:

1. What is this?
   Looks like some test score averager used in education. Peter Hopp
   The formula for computing grade point average is

   GPA =

   Σ(class GPA * class credit hours) Σ(class credit hours)

   The Ones index can be positioned opposite the number of class credit hours for which a GPA of 1 was scored. Next, the Twos index can be positioned opposite the number of class hours for which a GPA of 2 was scored, etc. The sum of these products is then read on the fixed linear scale in the center of the rule.
   There is a gauge mark on the top edge of the bottom slide, just above the 2.6 mark of the scale on the lower edge of the same slide. By positioning the gauge mark opposite the earlier computed sum on the log scale with the range 10 to 240, the GPA (Grade Point Average) can be read on the lower scale of the slide opposite the total class credit hours on the bottom fixed scale. Eamonn Gormley