1. This rule was produced around 1945 and was the first production rule from Pickett.
2. Because of its early date of production and the fact that it is considered the predecessor of the Nos. 2, 3 and 4 it is sometimes referred to as the Number 1
3. Compared to later rules there was a limited range of scales, no log-log for example.
4. What marked this rule out was the system of keeping track of the decimal point on the back. There were "mini" versions of the main scales (C,D and trig) repeated twice at either end on the back of the rule. By noting where results came in the "region number" and using the associated DN, RN-DZ scales it was possible to calculate the position of the decimal point in the answer.
5. There were also scales on the back for the number of decimal points for square and cube roots. By reading the number of digits to the left (or zeros to the right) of the decimal point  on the RN-DZ scales the number of digits in the root could be read off on the appropriate scale.

My thanks to John Mosand for the providing scans of this rule.

The back of the rule has special scales for decimal point keeping.