Transistor Can Packages TO-39 Case, Through-Hole
The TO-39 package is a through-hole device. A metal can, because of the shape of the semiconductor package.
TO-39 Transistor Lead Identification
TO-39 Semiconductors: [3-Leads]
PNP Transistor 2N3762 and 2N3763, 2N3743, 2N3866
PNP General Purpose Transistor, 2N3946
PNP High Voltage Transistor, 2N5415, 2N5416
PNP Low-Power Transistor, 2N4405
PNP Switching Transistor, 2N4033, 2N6193
PNP Transistor Amplifier, 2N4234, 2N4235, 2N4236, 2N5679, 2N5680
NPN Switching Transistor, 2N3252, 2N5339
NPN, Transistor Amplifier, 2N5681, 2N5682
NPN High-Frequency Transistor, 2N5109
NPN, Power Transistor, 2N4237
NPN, High Voltage Transistor, 2N3439
Note that the Collector is connected to the metal case.
The flat-top heat sink shown above is just one example of a press-on heat-sink used with round metal can transistors. Most if not all the other pages covering transistor packages show a different version. Although all the metal cans will use the same basic type.
Although shaded gray, most heat sinks will be have a Black Anodize finish. Many of the transistors listed above will handle between 5 and 10 watts maximum with a case temperature of 25 degrees centigrade. Although some may not be able to handle that amount of current.
The relative shape or size of the heat sink is not all that important, unless of course some other device might be in the way of the fins. The important characteristic is the thermal impedance of the heat sink. However the physical size and amount of contact area does lower the thermal impedance.