Hi there...
Billy, please try this link which is a pdf file from Eurocontrol
Eurocontrol pdf file
It shows the used aircraft abbreviations on flightplans in the Eurocontrol airspace.
They are calles BADA and indeed, you should call a B737-200 B73A, a B737-500 is a B73B and the NG planes are called B73C.
Greetings
Alex