Dexter,
The standard grading scale for circulated coins goes from P-01 ("poor-1") to AU58 (About uncirculated-58). The higher the number, the closer the coin is to an Uncirculated (or Mint State) grade.
AG means "about good." That grade has the numerical value of 3 on that 1-to-58 circulated coin grading scale. I think your coin is a little better than AG-03; I put it at G-06 ("Good-6") but it is still a heavily worn coin that is near the bottom of the scale.
The "90%" label probably refers to the fact that this coin (like all standard silver
US coins dated 1964 and earlier) is composed of an alloy containing 90% pure silver.
There are lots of informative threads in the Grading Practice forum. Go ahead and browse and you'll pick up a lot of useful information.
Edited by Jaobler
07/14/2008 5:06 pm