I came across a Italian Ebay advert. Based on this information:
COPPIA (per 2 sedili) CINTURE DI SICUREZZA VINTAGE ORIGINALI KLIPPAN COMPOSTE DALLE PARTI: 55707 - 14453/201 - 41457/2 MONTATE SU VARIE AUTO D'EPOCA TIPO FERRARI 400 308, LANCIA BETA SPIDER MONTECARLO, VARIE ALFA ROMEO E MERCEDES.
COME DA FOTO IN OTTIME CONDIZIONI. 1 PAIR SEAT BELT BELTS.
The Ferrari 400 seatbelts appears to be compatible with:
Lancia Beta Montecarlo Spider
Alfa Romeo Spider
Upon further checking I found out that Klippan is a Swedish company: http://www.klippan-safety.se/
Not completely strange as I think Volvo was the first car manufacturer using 3-point seat belts.