Just to note, I haven’t played since bowling had been released, so 0.1.6.0 was my first game of it. Before that, I decided to check out the bowling ball customization (because why not?). I noticed that I was able to select and use the Skull Bowling Ball despite not owning it (or any other custom ball, for that matter). It does not appear in my Steam inventory.
The Skull ball is available in the customization menu.
As you can see, I have not yet unlocked it.
Despite this, I can still use it.