Now that Janey's researched the ruler and given us links to its use, I only need to admit my inability to figure out how it would be used. Very interesting ruler! I'm like you, I love rulers and have quite a collection with noble intentions to one day use each and every one.
I have a ruler for setting border blocks on point and wow, when I used it the border was exactly the right size! Geez, I LOVE rulers when they work!!
I'd be interested to see something you've used that ruler for.