Interesting. I'm wondering how many molds were used to make that part too. It could be something like 1 machine in X amount has that defect where X is the amount of molds Singer was using. It could also be the crack only shows up after this serial number and is on every machine after it.
Apparently Singer didn't feel it was a serious enough defect to justify making a new mold to replace it.
Either way congrats on your new Rocketeer even though you're still short a case for your other one.
Rodney