That is a very interesting find indeed!
According to an article from Ruoteclassiche, the car was bodied by 2 Italian brothers, Umberto e Bruno De Mola.
King Baldwin's cousin, Count d'Assche, bought an Austin Healey 100, chassis #bn1/150649 in January 1954. It was soon bodied in a racy berlinetta by De Mola and it was used in some races and sold in 1958. It had a bad accident with a tram and was rebuilt by De Mola again, in the form we see today. Colour changed from green to white in that occasion. The car was restored at the highest level by Faralli Restauri.