>Chain-pin
I checked it and there was no pin out of ordinary. I found that the scrape scar exactly matches with the path of every pins on the lower-half of the secondary chain, though. mmm
>loose chain
The thing is that my A-bike's chain is not so loose...
I had to pull the chain very hard to expose it for the photo in my previous post in this topic.
I couldn't find any solid evidence of "stuck between the 'casing' and the wheel" in my A-bike at the moment. I'm checking it without riding, so the result might differ when the bike's on the move (or warmed up).
I also suspect that this could be because the rear wheel's axis or cog went off alignment, but not sure.