Hasselblad prototype A12 film back. The major change in the design of the Hasselblad film magazine was the modification made in 1968, with the introduction of the A12. The "A" in the designation was for "Automatic" since the new design made film loading more convenient, by automatically. Stopping at the first frame. The magazine here is engraved PROV 6, PROV designating Prototype and this being the 6th of the 10.
Produced for testing and approval. It is quite similar to the production item except the numbers on the counter wheel are smaller. And there is no serial number on the insert. This specific magazine was illustrated on page 192 of the Hasselblad Compendium. The magazine has been cleaned and checked, & functions as it should.