I always really liked the WM-51. The concept of built in headphones is cool by itself. And it also has a great design and the nice mechanism of this era with mechanical buttons that feel just right. It has auto reverse but no dolby and no bass boost. The battery compartment can take a single AA but also a gumstick battery, nice! It's a plastic walkman but it doesn't feel cheap. It's well designed and well built. I have two white ones but they also came in red.
The volume button looks like the old WM-20 style era, but the screw is different - a real screw this time.
Next to the nice button design - where the LOCK switch mechanically locks the buttons, including EJECT - I also like the design of the transparent area and the black area around it, not entirely oval but with a nice twist. There's a lot of attention to detail in this design and for me it's one of the better looking plastic walkmans.
Years ago I believed that there was only one model with integrated headphones, and a winding mechanism - the WM-51 that is. Then I learned about another model... and another... and now I own 4 different models with built-in windable headphones. I am not sure if there are more models with this interesting feature, if you know of another one let me know. The WM-F52 is next:
It has a similar - but not the same - mechanism as the WM-51 and from a distance you could maybe consider this as a WM-51 with tuner. However, it is very different. Way thicker and shorter, and where the WM-51 has the headphones exposed, the WM-F52 has a lid.
The backside has an interesting design. This walkman is also completely made out of plastic, but like the WM-51 it does not feel cheap like some low end walkmans do.
Unfortunately with this one, the headphone plastics have aged to the stage the plastic falls apart. The wires, speakers and winding system - which I would think of as the most sensitive part - all work perfectly fine, it's just that the headphone housing has detoriated.
Here you can see how thick it is, how it has the different mechanism with only one silver wheel, and that it has FWD/REV LEDs whereas the WM-51 only has a single BATT LED.
The 3rd model... a newer WM-EX348 Clearly a cheaper plastic model which feels lower end compared to the WM-51/WM-F52. Other than the integrated headphones there's not much special about this model. However, the headphone winding mechanism has a new twist: A spring that auto-retracts the headphone when you move the slides. Very similar to a vacuum cleaner. Sony calls this "Auto Wind Headphones"
We can recognise the button layout from the low-end models from this time, it does have enough features though with auto-reverse, mega bass and Dolby B.
I will close with this one, a WM-EX911, unfortunately in pretty bad shape and it lost its headphones over time.