1. As with many rules - this one went through a number of variations during its life.
2. This rule has an almost identical scale complement to the N3, the main differences being an absence of the guide to significant digits on the N3 and a different ordering of scales.
3. One feature of this series of rules is "back-to-back" scales. This is seen most clearly in the detail of back view where there are 4 pairs of log-log scales with numbers greater than one being paired with numbers less than one. For example the upper half of the N4 scale goes from 10 to 10¹⁰ and the lower half from 0.1 to 10^-10.
4. The rule is one of the few which does not have the A and B (square) scales.
5. The rule scanned is the second in the series. Among the differences in scales were:

Variation 1

• Back-to-back scales not used
• On the front face: No DF, CF, DI scales: L scale included.
• On the back face: No DF or L scale.

I have also identified two variants of variation 1. Variant 1A is probably the earlier of the two as it has the wording "Patents pending", though since none of the later variants mention patents this is not certain. The only other minor variant is that the CI scale uses black on white text on variant 1A and white on black text for variant 1B.

Variation 3

• Aluminium used (the other two were magnesium)
• Scales were given their "standard" names such as LL1 etc.
• This model had the designation N2-T

There were other differences in number of gauge (special) marks, placement and wording of company information and logo, and form of cursor. There was also a  variation which had the scale configuration of the 2-T which has the same configuration as the N2-T. The T was used by Pickett to differentiate white rules from those with the designation ES in eye-saver yellow. As far as is known, an eye-saver yellow version  was never produced.

I am grateful to Michael P O'leary (mpsoleary@earthlink.net) for providing information on the differences between the models and for the photographs used for the scales of variations 1 and 3.