Similar to the Glock V-MAG connectors I posted recently, GIS Tactical has a matching Glock stock available. While the stock is not currently on their website, I image it is made from steel in Hungary and TENIFER® treated like the V-MAG parts.
Shown at BSDA 2014 in Romania this year, this stock folds up to allow the pistol to be used more like a handgun. Or, folded out as an SBR (or a submachine gun with a legal Glock G18).
Below, the stock is shown on the Glock with a sling, deployed for CQB and folded up to be carried single-point styled.