Looks like a Maynard cartridge to me as well:
I jawjacked with a fellow shooting one of those at the range a while back. The casings he was using were new-production and made out of plastic. The clay pigeons he had set up at 50yards were getting broken with fair regularity, and he was mostly shooting offhand. Neat little carbines, although he was having ignition problems after an hour or so of shooting.