It is absolutely standard practice for this to happen. At contract completion, the contractor must destroy or return all classified materials: these sorts of designs would certainly be included.

It's pretty interesting, but a lot of this stuff is structured specifically so that the contractor organization doesn't retain knowledge that it's "not supposed to have", as wasteful as that ends up being. Even stuff like a list of personnel who worked on a project or production line might be included... so that a few years after the project is completed, the management of the contractor might not even be able to figure out who to ask, because they've destroyed even their knowledge of the list of people who worked on the component's design and manufacturing.