They push in, and there is a screw underneath - I've bough a few 6805 biathlon versions and swapped the flap for the tube.
Which website has the tubes for sale?
Not an aluminium stock, but a possible option? (not mine, by the way.)
http://www.ebay.co.uk/itm/Anschutz-8002-Laminated-Air-Rifle-Stock-/132125214888?hash=item1ec346c8a8:g:368AAOSwzgBYxuRq