I've noticed that there are different 20b engines (C, D and such...) What do the letters stand for/mean?

 

Higher letters (such as C, D) mean it is a newer version and has some improvements over the older versions. It also means that it will likely cost you more (D series will sell for more than an A series with everything else being equal).

 

d series are best aswell, they made various upgrades/fine tuning as they went on

 

Ohhhh I see. Thanks guys!