The law, as I understand it, apart from being an arse and all that, runs along the lines of: if the seatbelts weren't fitted as mandatory at the time of manufacture, then you don't have to fit them now. In other words, the law isn't backwardly compatible, which I would have thought came as read what with all the arse business and all that.
Seeing as your vehicle was built in 1945 you can also bribe policemen with silk stockings and legally shoot anyone in a Messerschmitt.
Donner und blitzen.