I agree it's for a Mauser 98. Don't know if you're looking for just something that it will fit or an "authentic" period/ manufacturer correct rifle for it. If that's the case, you'll have a good time trying to figure that out due to huge array of models and contract variations. Otherwise, short answer, Mauser 98.
Your bayonet is Czechoslovakian, for the 8 mm. VZ-24 Short Rifle. VZ-24's are pretty common. You should be able to find one at an affordable price.
Dude, that's nothing. I own a Lockshop/ Gun Store and people come buy a gun cause they find a box of ammo at home sometimes! I have had at least one guy who found a holster come& buy a gun to fit it!
Im sorry for your loss my friend. However, finding a period correct Mauser 98 to fit the Bayonet would be a great way to tribute your father in law. Best wishes
