They're the reference letters on the side of the bus stops themselves. Here is an example of D2 visible on mapillary: https://www.mapillary.com/app/?lat=52.70952531958&lng=-2.751562951049&z=17&focus=photo&pKey=1003308950201280&x=0.7740237754447985&y=0.5142085916555887&zoom=0
Will double check with the new metal bus stops.