I can't speak for all M39's, but in my sample size of 6 of them, half are wartime and half are post-war. All the wartime guns are tight at the muzzle end of the stock, and two of my post-war stocks are tight at the muzzle. One of my post-war stocks is factory free floated.
Bottom line, I think the finns went back and forward on this, perhaps depending on era or intended use of the gun?
One thought that comes to mind - in WW2, bayonet fighting was still doctrine and a tight muzzle fit was needed to ensure the stock wouldn't torque and crack from bayonet impact forces. By the 1960's when many depot-assembled post-war guns were put together, the rifles were generally only used for training and marksmanship, so maybe that's when they started floating them? Just a guess.