The TW-5B is like the TW-4 3" tunnel, but the WDC Modified Excluder adapter plate is screwed on. The purpose for attaching the adapter plate with screws is to provide interchangeability among holes. If you suffer SREH anxiety, or prefer trying to start a new colony with round holes, then you can use the TW-23 or a TW-23A round hole adapter plate until your birds have built nests or laid their eggs, then by removing 2 screws, switch the entrance to a crescent. Then, apply a small amount of latex caulk to waterproof the area where the adapter meets the tunnel. The TW-5B is for natural gourds, Supergourds, Troyer Horizontals, or Natureline gourds. On the Natureline, the TW-5B should be mounted on the back of the gourd. The TW-5B sells for $5.50 plus S&H.