It’s the original belt.
The battery wasn’t in it, thank god. No popped caps, no battery leakage.
The Twiggy Mac, along with its keyboard and mouse had been in its original padded carry bag at least since the original owner died in the 1990s. The original owner was a member of Apple’s board of directors at one point in time and when he died, his daughter ended up with it. I’m her computer tech and she called me over one day to go through “a bunch of old equipment” to identify what to donate and what to throw away, and this old equipment included the Twiggy Mac.