First, I am not an expert. I own an older A390 and a new AL390 (the "Walmart" gun, although I bought mine from Bass Pro). From the pictures that I have looked at of the 3901 it appears to have the rounded receiver like my A390. My AL390 (also marked made in the USA, with an Italian made barrel) has the semi hump back receiver. The A390 is a 30" barrel, and the AL 390 is a 28".
They both are completely reliable and I really don't notice the slight difference in the receivers. The 3901 is available in several different configurations. The Citizen has synthetic furniture and the Statesman has walnut furniture, but as far as I can tell, it is essentially the same gun as my older A390. In my opinion, you really can't get a better semi auto shotgun. And, yes, that includes the 391. The 391's "improved gas system" in my opinion is a solution to a problem that never existed. The 390s are much simpler and easier to break down and clean than the 391s.
I haven't had a chance to to measure the overall length, but 42" seems right. I will try to find time to measure mine.
