All attachable heatsinks are made from black anodised diecast aluminium.
Heatsinks in the FK 237 Series have the following features:

A very slim design ensures that it does not require much space
Easy assembly by sliding the heatsink onto the TO220 housing
Optimal heat transfer as the fastening clips on the TO 220 housing press firmly on the heatsink
Effective heat dissipation in both horizontal and vertical installation positions

FK 220-SA-220
No solder tags, with a transistor retaining spring for simple attachment to the TO 220 housing
FK 237-SA-220 H
With tin-plated solder tags, enabling it to be soldered directly onto the printed circuit board in a horizontal position
FK 237-SA-220 O
No solder tags
FK 237-SA-220V
With tin-plated solder tags, enabling it to be soldered directly onto the printed circuit board in a vertical position