Given that after world war II there would have been a lot of spent (or unspent) cartridge shells lying around in France just begging to be recycled into saxophones, it's reasonable to understand how this particular myth arose. Even if it was true, there is no reason for those instruments to sound any better (see above). However according to Wikipedia (the source of all reliable knowledge), although the brass was a particular alloy know as cartridge brass, Selmer's general manager Jerome Selmer has confirmed that Selmer never manufactured MKVIs from recycled shell casings.