The sights were supposedly calibrated for the bayonet being attached. Or so I heard on the interwebs.
I fired the Rumanian M44 with the bayonet extended and folded (it’s permanently attached) and could never get much of a POI shift.
The Finn M39 typically doesn’t come with a bayonet, since they are very rare. I’ve heard two stories about this. One, is that the bayonets were fabricated late war and most just went into storage, later to be scrapped. Second, is that, in their infinite wisdom, the Civil Guard thought the bayonet would be more familiar if it copied the Finnish puukko knife. Most soldiers, seeing the bayonet matching the knife already hanging on their belts, promptly ditched the bayonet, supposedly littering the forest floors with them.
I bought an aftermarket copy bayonet for the M39. It’s a nice trinket, but a pretty useless bayonet.