The TW-23 is a small adapter plate with a round hole. It has a single width exterior porch and an interior cling rod. It is designed to be used with the TW-5, TW-5A, TW-5B, and TW-5C tunnel for entrance hole interchangeability which is helpful if you are suffering from SREH anxiety or trying to start a colony and prefer to start with round holes. Simply remove the 2 attaching screws and replace the TW-23 with the crescent or WDC adapter plate that came with one of the tunnels listed above. The TW-23 sells for $4.00 each plus S&H.