Are NOFMA gradings applicable to unfinished domestic species only? From their website: "Unfinished NOFMA-certified flooring is categorized into four grades: NOFMA Clear, NOFMA Select, NOFMA No. 1 Common and NOFMA No. 2 Common, any of which will result in a serviceable floor." I couldn't find anything on prefinished flooring.

In a nutshell, yes.

I emailed them and received this reply:

"NOFMA does have grading rules for both unfinished product and factory finished product. However, currently there are no members who are producting NOFMA certified factory finished product."

Does anyone know why no manufacturer has an NOFMA-certified prefinished product?

All Bruce flooring use to say NOFMA on it. Seems like the Muskoka, I did last year had NOFMA on it, too.

I'm writing a term paper on hardwood flooring. In terms of NOFMA grades, the more I look, the more confusing it gets.

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For example, here its says Hard Maple, Beech and Birch are graded as NOFMA Select & Better, NOFMA #1 Common and NOFMA #2 Common.

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Here it says they're garded as First Grade, Second Grade and Third Grade.

This is not NOFMA, but if you go to the Maple Flooring Manufacturers Association (MFMA) website, it says "Random Length Maple Flooring" is graded as First, Second & Better, Third & Better, Third & Utility.

First, you are not allowed to post url's to other online vendors. Sorry, I don't make the rules so I suggest you edit and repost somehow. About those gradings. The first company is incorrect and the second one is correct.
The NOFMA and MFMA gradings are correct. For maple (I'm not sure about birch, hickory/pecan) the correct grading is #1, #2, #3 and you can find blends sometimes. Some mills will blend the 1 and 2 or the 2 and 3 and call #2 and better and so on.. On that first company's site, perhaps they were refering to prefinished floors, I don't know but those grading terms are normally used for oak flooring.
Here they post another error.

In fact, NOFMA was established in 1909, nearly 100 years ago. Way more than half a century! And MFMA was before them
Check out their sites.
http://www.maplefloor.org/literature/Strip_rules.htm
http://www.nofma.org/CertificationGradi ... fault.aspx
Mills will blend grades. Note that NOFMA has no designation on their site for "select and better" but it is a most common grade in oak strip flooring.