I now realize that part of my problem is that the diagnostic features I started with were not as useful at narrowing down the number of possible coins. Mint mark and Year, once I realized what I should be looking at, are much more useful than the ANNO, Bust facing, cross above features. If I had stepped back and thought about it for a moment that would have been more obvious.
My list of possible coins has been reduced to five:
SB 512, MIB 75D, 76D, 78D
SB 512, MIB 75D, 76D, 78D
SB 512, MIB 75D, 76D, 78D
SB 512 var , MIB 75D, 76D, 78D var (obv legend)
SB 512, MIB 75D, 76D, 78D
They all appear the same to me except for one is listed as a "var," which I assume means variation, and some have an "A" or "B" below. If I am understanding the terminology, that means the letter below the large "M," correct? If that is the case them I am left with:
SB 512, MIB 75D, 76D, 78D
SB 512, MIB 75D, 76D, 78D
Which to mee look the same. So I guess that one is wrapped up.
I appreciate what everyone is doing by not giving me the answers, I was just struggling to understand what and where the diagnostic features are.