I have read that the older black with red square and white "W" Wilson butt caps have trap doors. But I can't see where it is nor how to take it off. These butt caps were used on many older Wilson frames, lastly on the nCode 90's. On the K90's, it's a different black butt cap with a large silver "W" and it's obvious how to remove the trap door as there's a slot under the "W". Here are some pics of the butt caps I'm talking about that I found (not my pics). Do they really have trap doors and if so, how do I remove the trap door? The red square appears to be glued to the butt cap. Thanks in advance for any help.