[Hallicrafters] Hallicrafters S27 clarification of models

[Nick]

Hello all 

Being  a fairly recent convert to Hallicrafters I would be grateful if
some could clarify the differences between the S27 models. 

I have recently acquired an S27A (I think), the B version seem very
similar but with different coverage. The C on the other hand looks quite
different, and I've also seen reference to a D version.

So any pointers would be most welcome

Many thanks

Nick