[Seaplanes]

In the case of the He 51 it was more or less the same aircraft throughout, the only difference being the undercarriage, wheels for landplanes and floats for seaplanes. There could of course be minor updates to various equipment and installation of optional equipment such as bomb racks or rack for extra fuel tank. However, such changes did not necessarirly require a change of designation. The use of sub-type designations became more common after 1939. Again to use the He 59 as comparison, Heinkel called their He 59D version for just that, where as Arado called their He 59D for D-1. It was never a D-2. RLM and Luftwaffe used only He 59D.