The Vixen Optics EA 50.8mm to 43mm Adapter has a 50.8mm outer diameter, and one side has an internal 43mm female threading which perfectly matches the male threading upon Vixen's #3523 43mm DX Camera T-Adapter. Typically, Vixen's telescopes themselves have a 60mm inner diameter focuser barrel, and include an adapter which reduces the 60mm size to 50.8mm (2") so that you can use various 2" visual accessories such as star diagonals.
This adapter, combined with the #3523 adapter and your SLR camera's specific Vixen T-mount, allows you to image with or without an eyepiece in the optical path. If you choose to image without an eyepiece (called 'prime focus imaging'), you also have the benefit of being able to place a 48mm threaded filter within the optical path.
In some applications, the finished combination of (this adapter + the #3523 + T-Mount + SLR camera) needs to be placed further away from the telescope's objective than your focuser allows in order to focus properly. In this case, Vixen offers the #2957 extension tubes, which are simply a 40mm long extension of the optical path that you would place between this adapter and the #3523 DX T-Adapter so that you could focus the camera.