Hasselblad 16 (6x4.5) back. NOT the super slide version. Excellent condition, though I can't vouch for seals. Matching insert and shell. Film door spring is sufficiently tight. No wrinkles or bends in darkslide.
Not likely to be a prototype, but if it is I'll have to label it a collectors piece and raise the price.
I've had it for a number of years, but haven't used it recently. I have another 6x4.5 back and frankly don't need 2. I've never paid attention to the date code; given it's condition it's seen very little use. I took another look at the exposed gear and it shows no more wear than any of my A12 backs.
But, if this is indeed CT, then it should be a 16 and not an A16 back. In other words, it does not lock at frame 1. Also, the very reason I mentioned 1956 was because 'blads came out with this sort of gadget in '57.
Edit: The OP DOES mentioned this as a 16 back and not A16. My bad.