"Street Fighter II - The World Warrior" revision differences

What are actually the differences between the various revisions of “Street Fighter II - The World Warrior”?

I’m not talking about “Champion Edition” and “Hyper Fighting”. Rather, I’m talking about revisions like these:
“Street Fighter II - The World Warrior (USA 910206)”
“Street Fighter II - The World Warrior (USA 910214)”
"Street Fighter II - The World Warrior (USA 910318)"

Also, what I find interesting:

According to http://www.shmuplations.com/streetfighterii, the game was finished on February the 14th, 1991:
“Eventually all the bug checking and fine tuning came to an end, and the skies started to clear up. Yes, I’ll never forget that date, February 14, 1991 (Valentine’s Day!), at 7AM in the morning, when the final version was at last completed. “It’s done, it’s over. I can’t believe it’s actually done…” A line of tears streamed down my face. I took everyone by the hand, exclaiming “We did it! We finally finished it!””

This statement is confirmed by the fact that the first Japanese board actually has the revision number 910214. In MAME, this ROM is called sf2ja, with the letter A indicating that certain chips have the letter A stamped on them.

But for the American market, the revision 910214 is already the second version: sf2ub. While sf2ua is from February the 6th. And this is not some obscure prototype board, but a revision that’s actually quite common. (I alone own three of these boards with this specific revision.)

So, does anybody know why an American version of “Street Fighter II” that was created before the actual release is so common in the United States? Where does that board come from? I mean, there’s no Japanese equivalent. And they even bothered to update the revision letter:
910206 = version A in the US
910214 = version B in the US, version A in Japan

Can anybody explain this curiosity?