FYI - the TV in the 3150 is removable and designed to be taken outside to be hooked up. The bracket already on the TV is the mate to the outside bracket.
The 3150 TV also rotates to the bedroom, so there is no provision in the bedroom for an additonal TV. On the 318 for Hondo, the TV can not be used in the bedroom (doesn't rotate), so we installed an additional TV mounting bracket. That is why you got the extra mounting plate.
If you want an additional mounting bracket, contact Roger in our customer service department at email@example.com
Depending on shipping, they are normally about $30.
Hope this helps.