I believe it's one that doesn't have any other markings on it.

I'm Grizzly Industrial Hardware fan, but Harbor Freight or Grainger might have something too.

The keyword here is accuracy; you are inspecting a surface to determine if machining is needed to true it back up. <img src="/forums/images/graemlins/kewl.gif" alt="" />

Here's another; gives you a fair idea on how it's used for this application:
http://www.newwoodworker.com/reviews/wdpkrser24rvu.html

http://www.walkablecommunities.org/

Democracy is two wolves and a lamb voting on what to have for dinner. Liberty is a well-armed lamb contesting the vote.

**ubi apis- ibi salus**