This wasn't the case originally.
The story I heard is that Victor Hasselblad was testing out various prototype film backs to find a design that worked to his liking. Once he found the correct design, to avoid having people mix things up, he took a file and notched two V shaped notches and told his engineer to send the part out to be prepared for manufacturing. So, they measured the back to make blueprints and added the notches as part of the design...