Joe, I could not imagine -- my son has changed out the belts on my 328K when required. I'm not sure of the number, but I searched my original manual, and it has NOTHING in there regarding the belt! I suspect Singer expected that we would take the machine in to have the belt changed! FAT CHANCE! I think the belt we have is a round stretchable belt and it has been on there a long time, presenting no problem! My machine is a solid hand/balance wheel! The belt I have is about the same thickness as the bobbin tire!
Wish I had more, but I don't! Glad your machine is working okay!
Jeanette