Picture of the mount and sensor of upcoming Nikon mirrorless camera named "X810" leaked on the Chineses forum Xitek:

Brief translation (thanks B.): the new mirrorless demo machine is here, temperatly named X810. The user is not going to post pictures of the whole camera due to trade secret and he do not want to lost his job. but he did post a picture of the mount as requested by other users. As we can see it is not a F-mount and the senser size seems to be 1/2.3 or 2.5. many users seems to be disappointed with the small sensor it has.

The mount on the picture is similar to various, previously filed patents from Nikon for a mirrorless camera: