Thanks.... it turns out that the bobbins the DGD's DH got were so snug in the bobbin case that they wouldn't turn. So when the thread had pulled enough that the thread on the bobbin couldn't move, the thread would break.
We went to the store where he had purchased them.... he thought they would have different brands. It was an experience. The sales person first told me that I needed a different bobbin type.... she thought I said a singer 221 instead of a 201. Then she told me that plastic bobbins should work. We told her about the bobbins coming apart when the DH tried to get them out of the machine.... they were that oversized and stuck. She said she didn't think that would happen. We had the bobbin tops and bottoms to show her. We told her we only wanted metal bobbins and she started telling me about the plastic bobbin case. I said, there isn't any plastic in a 1942 Singer 201 and she told us we were wrong. She had seen 201s with plastic bobbin cases..... Oh my!
We went to a different store to get bobbins..... she didn't know what she was talking about even though there were some nice vintage machines on high shelves around the store.
The machine is working fine now.... no bobbin thread breaking..... how do those people keep their jobs??