Your package rifle has the Axis style magazine, whereas the Choate stock is designed for use with the metal bottomed magazines and accompanying metal surround/retention plate. Good news is that if you really want that stock you'd just need to buy the metal bottom plate for the magazine and the surround/retention plate from Savage. Might be a little pricey though as I think the surround plate runs around $130-140.
And no, the SSS extensions won't work for your setup either.
Basically Savage has created yet another mess by using the Axis style magazines on their package rifles as it's creating a ton of confusion such as yours. Savage doesn't care though - it lets them save a few pennies in manufacturing cost so they can have a higher profit margin.
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