Transistor Outline (T.O) Packaging
Entirely made of metal with microchip mounted on its metal can base and then covered with a metal cap. TO-3 is designed to accommodate large heat sinks to dissipate heat and increase power handling capability.
The tab is located 45° from pin 1. The lead diameter is nominally 0.45 mm, the diameter of the cap is 4.5-4.95mm (0.178-0.195 inches) and the overall width (not including the tab) is 5.4 mm. Ref. wiki