I think it's the aerial for using the FM radio without headphones. Sony used a similar one with the ICF-S11W (and probably other models too).
If an earphone or headphones are used, the shielding in the cable connects to the common ground in the radio and acts as an FM antenna. This cable does the same without stopping the sound coming from the speaker as an earphone would do. But I don't have such a cable so I don't know how the jackplug looks like. All pictures I could find show this cable connected.
Wegavision is at it again, another treasure trove of uploads from Germany! This is Audio Magazine (Germany) from 1983. Oddly this seems to be after the stereo microcassette craze ended in late 82. Sorry can't read it but the photos are cool! I'm amazed at how small the component tape decks are compared to the portables.
This showed up on USA Ebay, silly money but these things are rare! I looked up Emly Starr, she's Belgium but she won a Japanese Singing Contest in 1980, she was probably pretty hot when these were made around 1981. https://www.ebay.com/itm/314277153556
just-cassette has some great photos of the M-950 https://www.just-cassette.com/post/sony-m-950-microcassette-corder I wonder if they infringed on Olympus "D" System Patent for modular players?
Stereoplay 1979, sorry can't read it but I love the set! The Olympus D System with a few attachements.
Well, that microcassette fever (fieber) was relatively shortlived because I bought a new Olympus SR-11 in 1984-ish for 135 Dutch Guilders, that would be about 120 Mark, so it would seem that model was discounted quite a lot in a reasonably short time seeing it was 450 Mark in 1983 according to the article.
It looks like it lasted maybe two years but died pretty quick. I don't think the small size was enough to convert the compact cassette people. Add in the cost was pretty high to get into this system.
A shop in Japan was blowing out the Sanyo X-66 in 1983, I have to believe these were unsold overstock since the stereo microcassette trend was just about dead by 83.
The Japan Times 1982-07-09 The AIWA HS-M2, it looks like it was called the "Cabin" in Japan, interesting!
I just found this the other day, a National RN-Z36. Didn't know this existed before, it's a really nicely designed unit with an interesting speaker "dock". I really like the 80's aesthetics of the slanted lines and surfaces. This'll go very nicely on the desk with a Toshiba XR-P9 discman. I expected the speaker to have special connection points build in like many walkman-boombox combos, but this one just hooks up to the unit with a regular cable. Interestingly, the speaker does not have a separate model number.