It is a real 'accessory' and is not treated as a firearm. They are illegal in California, since the resultant combo is illegal. I saw several at a recent gun show and it took the guy about 12 seconds to convert from pistol to carbine.
The one's they had there were the 1911 version, but the Glocks are available now as well.
From Mech-Tech Systems
For the 1911 fans...
Note: when you see a picture and wonder where it came from, on a pc, you can right-click the image and choose 'properties' and it will reveal it's source.