"No Maker" M39 aka "Sneak" Rifle

So I recently purchased a M39 dated 1967. Very good condition, and I can't wait to shoot it. I've done some research on these late production Mosins. I've read that there are a number of names they are referred to, and that while "no maker" M39s or similar is the preferred nomenclature, "sneak" rifle is also somewhat common (and also disliked by collectors). There seems to be some story behind where this name came from, but from what I've been reading, I haven't found anything especially clear on the origin. So does anyone know the story behind this name?