This shows the attachment of the frame extension. As you can see, there is actually a slot milled into the frame for the extension. This gives a lot more purchase area for the silver solder to join the extension to the frame. I have seen an example of a nicely done extension make-up that did not use this slot. If the carbine was ever fired, this "pasted on" extension would surely fail!
http://boards.rennlist.com/upload/Extension.jpg