Symbol on an old slide rule.

I have an old wooden slide rule with no maker's name.
It has the symbol for pi,but it also has a symbol marked at 2.06,3.44 and 6.38 (subject to the accuracy of my reading the values).The symbol looks like O with a line slanted underneath to make the whole thing look like a slanted P and the O has n just above it like a power.
On the back of the slide rule there are tables for weights/cubic foot for different substances,and strengths,tension,compression and shearing.
Any ideas?

Originally posted by jb70
I have an old wooden slide rule with no maker's name.
It has the symbol for pi,but it also has a symbol marked at 2.06,3.44 and 6.38 (subject to the accuracy of my reading the values).The symbol looks like O with a line slanted underneath to make the whole thing look like a slanted P and the O has n just above it like a power.
On the back of the slide rule th ...[text shortened]... /cubic foot for different substances,and strengths,tension,compression and shearing.
Any ideas?

Originally posted by Shallow Blue
Sounds like the p' and p'' marks, as described (albeit, AFAICT, slightly wrongly) here: [url]http://www.sphere.bc.ca/test/scales.html[/url] and here: [url]http://sliderulemuseum.com/SR_Terms.htm[/url]. The third one I don't know, but if it's 6.28 rather than 6.38, it's 2pi.

A picture would be useful.