Can anyone explain Pioneer model numbers?

I have been looking at various Pioneer DVD Players and come across a whole host of model numbers....565, 545, 575, 585, 989, 717, 747, 656, 668, 989, etc. etc.

I assume there is some logic to this, such as the first and last digits indicate position in the range, such that 747 is a higher-spec than a 545. Maybe the middle digit indicates the version, such that a 747 is the replacement for the 737.

Am I on the right lines here? Not all model numbers fit this scheme and would really appreciate some help!

I guess that the underlying question is whether I would be better going for an older, higher-spec machine versus a newer, lower one.