What Is This Device? (Image Alert)

I see the cage is a MM jailbird model, but does anyone have an idea what that snug band is? Maybe it’s some sort of anti-pullout feature. I spotted this on Tumblr and haven’t been able to track down any details about it. Image

"it's a big piercing tunnel that I use inside my cage as a frenum shield. It gives total sensory deprivation"