I think it just signifies that they(Mossberg) just made some minor changes to the model and the B identifies it is the same as the A, but with those changes. Not positive, but they do that with other things. The platform is the same but they add different options so to speak.
I just gave someone else with a Mossberg question these to links. I've included them here for you also so you can call or email if you can't find any info at wither website. At the Mossberg website, check the discontinued models to see if they are even still listed and go to the specs and see? If not, go to the Mossberg collectors website and see if they have any info.
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