What expansion is this?

[What expansion is this?]

So I am working on a Atari 400 I picked up at a rummage sale (I am new to classic Atari systems) Blank screen so I do a little research and pop it open and find this board inside. It is connected to a SPDT switch that needed to be replaced. I got the 400 working but have no clue what this is or what the switch does. Any help is greatly appreciated.

[Vect-Rex]

Guys Games and Beer has been working on a big project for the Midwest Gaming Classic. Check out the video. Big shout out to Arcade Jason for inspiring this project. https://youtu.be/wj-PueSRgFM

[RCA Studio II Multicart]

I would like to purchase the Multicart and a shell and box when they are available again

Thank you!