Delta originally had a lot more categories you could bid. This included having a domestic and international category of the widebodies. Part of this was likely because at the time it saved a lot of money on chart distribution to not give global charts to everyone.

There was a 767 category (covered 757 as well). The international version was 7ER. When they combined the categories they kept the international designation (for some reason). Hence 7ER was born.

The 737 category was the classics. Later there was a 73G for the glass cockpit 737-300s ( I don't recall if there was an additional category there for 737-300). When the 737-800 arrived they were given the 73N category for NG. Even though it's the only 737 category now, Delta seems to almost never change category names (A220 seems to be the only exception).

So when you see 73NB it's not NB at the end, it's 73N with B at the end. As already explained, A and B are seat positions.