U = Upshoot

The upshoot refers to the bottom part of the posting aperture that projects internally into the box. Some early boxes had a downshoot and posted mail dropped straight into the body. This was changed to an upshoot design, where mail had to be pushed up before dropping into the box; this was thought to offer better security.
The Letter Box Study Group
2016-02-10T16:11:46+00:00
The upshoot refers to the bottom part of the posting aperture that projects internally into the box. Some early boxes had a downshoot and posted mail dropped straight into the body. This was changed to an upshoot design, where mail had to be pushed up before dropping into the box; this was thought to offer better security.