There are several versions of bobbins.
A: The first was a one piece bobbin turned out of solid stock on a lathe.
B: Next was the two piece unit stamped out of sheet metal then joined in the middle.
C: Then was the three piece with a solid middle.
D: Followed by the three piece bobbin with a rolled middle. I've seen some that were welded and some with the middle just haphazardly rolled to a tube shape.
E: Plastic ones.
All of the three piece units are put together by the center tube being inserted into the disc then the end flared over.
I've not had any fall apart yet, but I've had them crooked and warped right out of the box.
I love it when I can come up with the older ones. They work really good.