Blue Book online has info that differentiates the different models/grades, but I don't consider it authoritative, or necessarily complete. You can check it out by registering, and subscribing for a month. It's $3.95, I think.

 

Grade "I" is the basic grade which has the least amount of engraving and lowest quality wood. It goes up progressively to about Grade VI with each increase in Roman Numeral being a little better than the previous (lower) number. Every model may not have been made in every grade. In other words, for some models, they may have made only one or two different grades. Sorry that I can't be more specific.

 

Give the customer service folks at W'by a call with the ser. no. and they can tell you what it is. Even though they moved out of Kali, there are still a fair number of the old timers there that can answer generic questions that aren't ser. no. specific.