This skull is compatible with both the Astronaut and Diver keycaps.
For the Astronaut this skull is one of 3 parts that make up the keycap. Printed in frosted detail plastic this traveller from the stars now peers out from his cracked visor.
You can order the other 2 parts of the keycap here:
For the diver this skull is one of 2 parts that make up the keycap. This underwater adventurer remains locked within the helmet that once kept him safe. You can order the diver's helmet here: