Chrys at Needlebar told me this about serial numbers:
"As Carol pointed out the Singer charts have some errors. The charts also have some changes from the first time they posted them. The models that say St. Jean were shipped there for final assembly. The heads were cast at Elizabethport. They did that because St. Jean was behind on production."
Chrys told me that sometimes a run of a certain model of machine was stopped to make a short run of another. Does your machine have the characteristics of an early FW model?