Some early shipped XStation Mount designs were accidently printed in the first run of XStation mounts, we do not have a total count of these units. To determine if you got on of these you will see that the top doesn't quite fit over the mount. If this problem comes up on your install please submit a ticket and we will get a replacement to you.
For the replacement you will need to transfer the extension cable to the new mount by undoing the bottom 4 screws and moving the parts over to the new mount.
If your mount has AJ3 5XXX on the bottom it is the current version and will fit perfectly fine.