1. This is a mystery electrical rule with no indication of maker's name. It does however have a number of unusual features

2. The rule has the electrical scales on the front of the rule. Normally they are in the well of the stock. The scale for the sectional area of the cable is in mm so the rule was probably made in continental Europe.

3. In the well of the stock there is a number of "29". This could be the year of manufacture - but I have never seen a year of manufacture in that position before.

4. The cursor is also unusual. It is of glass with brass strips at the top and bottom and (this is the unusual feature) small, fine  metal strips at either side of the glass joining the two brass strips. One of these is missing on my rule.

5. The rule uses a Perspex window for the reverse of the slide with a line for all three scales. The data strip on the back has been removed and replaced by hand-written guides on using the rule relating the position of the decimal point, trig scales, squares and square roots, and cubes and cube roots. The cube scale, unusually, is on the slide.

6. The construction of the rule has three screws on one side, presumable for tension adjustment. On this side there is an inch scale so it was presumable intended for the English market.

7. The range of the log-log scales is unusual, from 1.07 to 2.0 and 2.0 to 1000.

7. The symbol on the box, which I have shown as scanned and my interpretation of it, is also not found on any of my other rules.