Somehow I've missed this thread. VW I've sent you a PM about info for the registry.
I agree that your chasis is a '56 (Rick, you got it backwards.....the bracket is on top in '56 & on bottom for '57 & later models). It's also possible that that older motor was still on the shelf & got used in '56.
My opinion is that Simplex kept so many parts (like complete engines) in storage for service orders. After a certain amount of time, if the parts wern't sold, they got rotated back into production and newer parts took their place on the shelf. This would be one reason for the wild delta's between frame & motor numbers.
The really cool part of this bike is that it is only 1 frame number newer than one already in the registry (# 15214, owned by JamesC). We need to get some more info from him to see how close the bikes started out.
Mike Sal