Jeremy, your frame number is only 7 away from another known '57 bike (#16315, referenced by Bill Erickson from Pat Williams list). This puts it 1,073 frames after mine was made. From your frame & motor numbers, I'd say there's a solid case for your's being a 1957 model. One thing to keep in mind is that Eric told me there were several left over '57's which were sold as '58's.
Mine on the otherhand....I'm now thinking my engine has been replaced with a later model one (the engine number, 23561, is just too far down the road for my frame number). Who ever did the swap did a good job & re-used all of the original hardware. I can't find evidence of any nuts or bolts being replaced with other types.
My bike has the stubby tail pipe version muffler. In reading the Wollard book, it sounds like the left pipe on the dual pipe version was just a dummy pipe.....maybe one of the other guys can chime in & confirm or deny this. The book also said there is a restrictor at the back of the right side pipe.
Mike Sal