The science-fiction-like world of 3D printing is starting to get some very real attention from Wall Street. This week Jefferies initiated coverage on the sector with “buy” ratings on three of its five major players – 3D SystemsDDD +3.89%, Stratasys and ExOne – and “holds” on the other two, Arcam and voxeljet. These companies could revolutionize mass manufacturing, Jefferies says, and could even get competition from a bellwether of the traditional printing industry: Hewlett Packard.