To Flir: They are all essentially the same engine, but the reason for the A,B,C,D and E designations has to do with the type of fuel control (Fuel Management Module or Fuel Metering Unit), output shaft speed and angle, type of engine management (Data Collection Unit or Q and MGT Trim Boxes), etc. Eurocopter's engines will always be different for the 135 because the output shaft is at 26º in that ship.
Application and takeoff Horse Power is as follows:
206A 621 HP MD900
FMU type fuel control
MGT and Q trim boxes
206B 431 HP EC 135
Similar to 206A with
FMM fuel system
DCU instead of trim boxes
26º angled output drive for EC application
Lower output shaft speed
Fire detection and engine indicating hardware supplied by Pratt
206C 561 HP A109, Kazan Ansat
Similar to 206A with
FMM Fuel Control
DCU
207D 710 HP B427
FMM and DCU
Shrouded FMM
Engine indicating hardware supplied by Pratt.
206E 646 HP MD902
Similar to 206A with
FMU
DCU
Upgraded fuel pump
FCU shroud.
This list is a couple of years old so there will certainly be changes now, but you get the picture.