Keuffel & Esser 68 1892 (4158) 10″ Plastic Mannheim Beginner’s Slide Rule 68 1892 10″ Plastic Mannheim Beginner’s Slide Rule – MHF-3568 1892 10″ Plastic Mannheim Beginner’s Slide Rule, Desktop Version – MHF-3668 1892 10″ Plastic Mannheim Beginner’s Slide Rule, Desktop Version – MHF-37 See Also: 4158 10″ Plastic Mannheim Beginner’s Slide Rule