Thank you Martinue for opening this tread, and the willingness to share you knowledge with others.
Leonibus and Consten raced an Alfa Romeo at Le Mans in 1960, they participated in the class 1,1L. Probably an Conrero prepared car and engine. I don't know if it is the same car, but the connection to Conrero could be a tread.
If I my ad some pictures from the Conrero book, unfortunately I don't have the chassis numbers, but maybe someone can fill those in .
First picture: Carlo Peroglio at Zeltweg in Austria, the 2 pictures is from the same race, look at the numbers,69, at the side of the white Giulietta. It must be at the start. Unfortunately no year given
Second picture: Carlo Mario Abate at 8 Rally del Sestriere. No year given probably 1957.
Third picture: Goccia at la Aosta Pila, probably 1960
Forth picture: 1959 De Leonibus at Passo dei Giovi.
Fifth Picture: 1958 I think this is interesting as its given as Conrero 1600 GTZ, from Liege-Rom-Liege. Driven by Bernhard Consten and Herbert. Note the text on the Marchal add.