I found a Nylon 66 with SN 21147xx, my 1st .22 rifle, under the tree on Christmas 1969. I've probably shot 20K rounds through this rifle with only one spot of trouble, when I tried to shoot some of that wax covered Russian Junior-brand ammo in the sea-green boxes. With any good grade of ammo, the Nylon 66s were damn close to 100% reliable.