I realize this post is old, but I just thought I'd throw it out there in case it's still relevant to the poster...
The Tac-13 uses an adapter shim to allow it to accept the Raptor birdshead grip (which is 870 pattern). I suspect (but can't confirm) that the factory 870 sidefolder would bolt right up to the Tac-13 as long as the adaptor was left in-place.