Yep Super Dave is correct on the strong/weak tailfeathers on 7/8 varieties, but as to the "B" never seen that on a slab after the grade, are you sure it's on the actual paper in side the holder and not a sticker or written on the outside of the slab? A * or + is common as are some other designations like FBL, FB, FS, Full Bell Lines, Full Bands, Full Steps, etc.
It could be part of a variety designation like a B1 Rev possibly but there should be a VAM
in front of it on a Morgan or Peace dollar
"Buy the Book Before You Buy the Coin" - Aaron R. Feldman - "And read it" - Me 2013! ANA
Life Member #3288 in good standing since 1982, EAC Member #6202, NBS Member, 2¢ variety collector.