Not much popping or backfiring, unless an ignition goes bad. The double nutting helps a bit to retain position as the wood of the prop compresses. I'd be willing to bet the 50cc came with two nuts.
As fas as where the shoulder engages, the stud needs to be screwed in far enough to get plenty of threads gripping inside the engine's crankshaft. By shoulder I guess he means on the crankshafts tapered end that the hub mounts on, and the stud screws into. Unless you remove the hub you can only guess where that is.