Hi Jerry.
The dashed line is a standard way, in mechanical drawing/drafting, to
represent something which is not visible from the current perspective.
In other words, when looking at the bottom of a TO-3, you can see the
pins, their recesses, and the mounting holes.
The dashed lines (as Dave and Dave said) indicate there is something
on the "other side you can't see from the bottom view. In this case
the raised "top" portion of the case containing the actual die, wires,
etc.
- Mark
