That is exceedingly weird. The manual I have shows the AK3 plate in use, and the upper threading, but that's it. There is no mention, like you say of other parts. If I thread according to the instructions, and change the plate, it works, but I have a different bobbin case than you do, so I'm not sure what to tell you. Does the round ring fit in where the bobbin would go? Could the slide plate close with it in place?
Might be from a toilet...