You may want to remove #11, look like it doesn't exist.
From the Magnavox Odyssey FAQ
Also, there is a mysterious cartridge #11. One could think that it was designed for an unreleased game. This is false: it was planned for Basketball, but then cancelled and replaced by cartridge #8 (used with Handball).
And you may want to add the "Apex-Magnavox blue card "
This may be rarest Odyssey add-on part. The blue card is a 2-in-1 double-ender cartridge which combines cartridges 7 and 8 for playing Handball and Volleyball.