Did the gun shop accidentally sell me an SBR?

[STI]

always ask for and get the real information on odt to cover your ass. SMH

In

 

[GAgunLAWbooklet]

Got it. I would say the solution would be to either fabricate a stock for it or take it back and demand a refund.

Yep, and if you fabricate a stock for it, keep in mind that it doesn't have to be a very long or comfortable stock.
It does not even have to be a "stock" --some kind of extension on the back of the receiver could extend the weapon's over all length beyond 26 inches.
I'd consider some kind of mounting bolts or a set of prongs on the back of the receiver that would be more or less permanently mounted there,
and they themselves would increase the length to over 26 inches.
Onto those you could slip a homemade stock consisting of a couple of tubular rails and a block of wood for the piece that presses into your shoulder.
You wouldn't have to "use" this stock; as long as you have it in your possession with the gun I don't think anybody would question you.