I got this bike used from an English teacher who was returning from Japan to New Zealand, it cost 3000 yen (~ $30) and I used it almost daily as my transport from my apartment to Chikusa Station and back.
Since the time I got the bike it had a bell and a weird black plastic block on the handlebar. I was not sure what the plastic thing was for but I also could not remove it because the screws were rusted and I did not have the correct wrench. Today, while biking from my apartment to dinner, I realized the black plastic block was for holding the cable lock. I use to just hang the cable lock around the T in the handlebar.
Honestly, I'm not sure if this is the actual use of the black plastic block, but it seems to work well.