My Dad has an antique revolver shoulder stock. It's an iron frame shoulder stock, with a cup that the revovler grip would sit in, with a little wing bolt that gets screwed in to hold it in place.
Constructive possession? I don't think so. He has nothing it would fit, and never did.
Not a felony to OWN such a device....but it IS a felony to attach it to a firearm, without an SBR. Even if you don't ever shoot it that way. Even if you attach the stock for 10 seconds in the privacy of your own home, then remove it.