Amazingly, in the manufacturing world, the only person who might know the final product is the lead engineer for the project.
in the world of rapid prototyping and one off production, it's entirely possible to keep everyone in the dark, particularly when you're talking about only 3 components being major.
your enclosure is easy to control, limited production and prototypes can all be done in house.
circuitry is easy, you're connecting components, not making something truly tangible.
software is easy, you're working on an interface not unlike one that all ready exists.
it's the handful of people who are bringing all of the pieces of together that you have to truly control.
It's referred to as compartmentalized information and production, and it works for most major governments.