Here is another example of this unknown 1956 Unknown that confirms this is not a frankenwatch but a factory model. The case is a three piece with a case back and case ring. The inside case back is marked US ZONE GERMANY. The movement is an 11ACC 17J with an L6 date code. Great looking waffle dial with even hour markers. No ads to match this one. Looks like a Senator case so it might be an unknown variant of the Senator. Unknown, like the other, for now.
Everything points to a unique model to me, especially now that we have a second example. I think the US Zone Germany and 5 digit serial number suggests this was a special model. It is possible it is a version of the Navigator made in the US Zone Germany with a German 3 piece case. In absence of any ad or linebook confirmations I suggest unknown is the best option like the other one. Or do we set up a new Name.... Bulova US Zone Germany to distinguish these unique models.