Sony WM-EX Flagship models
Sometimes I found it interesting that model numbers actually mean something. You can sorta tell high end models by just looking at the model number. Here are some of the patterns I noticed on Sony Walkman models. The number 4 is considered unlucky in Japanese culture, its pronunciation sounds like the word death. So, they usually avoid using the number 4. The other thing I noticed is that Sony likes to use odd numbers to represent their high end models. I could be wrong but here are some of the examples. Also for portable cassette players, if it supports Dolby C, it is most likely a top end model. This applies to all brands.
Sony cassette Walkman:
WM-3, WM-3EX, WM-7, WM-F5, WM-D3, WM-EX1, WM-EX3, WM-EX7, WM-EX9, WM-701C, WM-707, WM-DD9 etc.
Aiwa: HS-PX101, HS-PX303, HS-PX505,HS-PX707, HS-PX1000, HS-JX707,HS-JX505,HS-JX929 etc.
D-5, D50, D5MKII, D-555, D-777, D-303, D-311, D311, D-EJ50, etc.
MZ-1, MZ-EH1, MZ-NH1, MZ-NH3D, MZ-RH1 etc.