That is correct. Neither fighting knife uses the M3 or M7 or M8 scabbards. They have a peculiar scabbard made for them. Even if the Kbar, Mk2 knife will fit where the M1bayonet had been, it's not correct. Have a look at Argo knives and he'll sell you a correct blade with correct scabbard. Examples...
https://argocoins.com/product/ww2-rc...ighting-knife/
https://argocoins.com/product/ww2-ge...hting-knife-2/