From Wiki

Sterling No. 689 Precision Senior Mannheim

• Type: Open Body Enhanced Mannheim
• Size: 10 Inches
• Material: Plastic
• Scales:
  • Front: A [ B CI C ] D K
  • Back: [ S L T ]

A selection of three:

• HvH 3938: White body with green slide. With vinyl sleeve case.
• HvH 2391: White body and slide, all black lettering.
• HvH 3381: White body and slide, red lettering on slide.

Unlike a traditional Mannheim, the reverse of the rule is not solid, but pierced so you can see the reverse of the slide. There is no mark against which you can set the S and T scales. You can scratch one on yourself, or flip the slide over. Read S against A and T against C.

The label on Rule #1's case suggests Sterling Plastics is a division of Borden, Inc. Evaporated milk and slide rules. That's diversification! Rule #1 also comes with an instruction sheet, printed on both sides that claims it's "A complete course in use and operation of the slide rule," and that this slide rule is "A quality instrument for student of professional." The label on the case gives a copyright date of 1972. Let 1972, then, be the year the slide rule died, regardless of the year we actually noticed.

Compare with the Official 98-1114. What's a "Senior Mannheim" do you suppose? The point of sale label suggests this is the model 589. Confusion ensues.