First, watch the linked video to get the scope of the leaks so far, if you haven’t been tracking them already: www.youtube.com/watch #videogames #nintendo #switch2 #leaks
If that is a PLUS button on the tablet side, that “button” looking thing on the top of the S2 Joycon could just be a lever to actuate it when the Joycon is connected. There may be added functionality for using the tablet without the Joycons which the PLUS and MINUS buttons could facilitate.
The ovoid next to the right shoulder button could just be a cover for an access screw. Another leaker mentioned that the shoulder buttons were the actual magnets? If so, then the slot/tab arrangement is likely a physical connector.
As for the source of the leaks, it …