1. Was produced by Davis of Derby (UK). Though the maker's name does not appear on the rule.
2. The rule has several carefully hand-marked gauge points.
They are (with explanation where I have been able to identify them):

• HP/CV : 1.103: British horse power (HP) and French horse power (cheval-vapeur, CV).
• BHP/KWT: 1.34: British horse power per kilowatt (.00134)
• Kgm: 2.21, pounds per kilogram
• M/M: 2.54 (inches per centimetre ?)
• C1: 2.60 : This is not the usual C1 which is marked as a standard.
• ST: 2.80
• GM: 3.10
• M/FT: 3.28: Metres per foot
• M3/FT3: 3.53 Cubic feet per cubic metre (35.3)
• M/IN: 3.93 : inches per metre (39.37)
• CAL/BTU: 3.95 : Calories per British Thermal Unit
• FT/AT: 4.31

3. The rule has a metal back with "windows" for the trigonometric and logarithm scales.