Walkman D3

excellent observation

No idea but an amazing observation! I have two WM-101's with some very interesting omissions and additions as well

... maybe it is just used as a stabilising diode and its light is not fix because the fluctuating current, that is why it is not visible.
it is just guessing. we have to see the schematic

As other members have been saying, good observation from member ElisioClio about the covered-up "red battery light." Maybe Sony chose to have this Walkman model of theirs to not have a "red battery light" showing in the front panel for the user. It's known to audio-system collectors and to technicians that scores of models shared the same internal components: integrated circuit boards (such as above), DC motors and so on. The façade -- usually less of a budget-buster for the company -- will state the features and will have controls of those features that vary from model to model. Our man Brutus hinted at that widespread "shared interior, different exterior" practice when he posted about having "two [Walkman] WM-101's with some very interesting omissions and additions as well."

@Easthelp

Hi. Yes, I do not need a led bulb. Or, a led burned and exchanged a led and on the other hand ...

According to the Service Manual it is a technical thing and doesn't have a display function.
The LED is marked "voltage shift". As far as I can see it has a control function for the LED-drive IC, but no stabilization functions.
It will light in BATT position, but this is just a technical "coincidence".