1. This is a five inch version of the 378 electrical rule. The scale arrangement and use of gauge marks is identical. The only difference with the 378 is that this rule has not got the marks "Quot +1" and "Prod -1" used to aid fixing the position of the decimal point.

2. As with many other electrical rules, the slide has a protruding "teeth" which function as a cursor for scales in the well on the stock.

3. On the back of the rule there are small marks on the "window" to all the trigonometrical and logarithmic scales on the reverse of the slide to be used without need to remove it and turn it over.

4. Later electrical rules had a scale for variation of temperature with resistance which this one does not.

5. It normally easy to date Faber Castell rules as on the back they have the date and month stamped on them. My 378, for example, has an 8 for August and a second  8 for 1908. This rule has a single "0" in the month position.