sorry that I couldn't find the time to respond earlier
I remember a discussion at the TNF where the circuito delle tre province was also touched: Atlas F1 Bulletin Board - Old Italian tracks
And here: Atlas F1 Bulletin Board - Old Italian tracks
Here is a link to gdecarli's site: CIRCUITI - TRACKS
He collected all availiable infos about the track.
And I saw a photo of Mario Tadini in his Alfa in Angela Cherrett's book about the 6C1500, 1750, 1900.
On page 159 we see the 1750SS of Tadini underneath a banner reading "Automobile Club Bologna Circuito Delle Tre Provincie (Coppa Porretta) Porretta 1.9.1929". He finished third and surely it's interesting to note that his car carried #33.
What I found most interesting about the 1931 edition of the race was that this was Enzo Ferrari's last race. Please correct me if I'm wrong!
In 1931 Enzo participated in two events, in the Bobbio-Monte Penice race on June 14 and in that race on the circuito delle tre province in August.
Reason for him to retire from active driving was the birth of his son Dino in January 1932.
More photos of the event can be found in La Scuderia Ferrari 1929-1939 by Orsini, Zagari. See p22 for a photo of Ferrari-Verdelli and on p111 there's a photo of 1931 winner Nuvolari with his Alfa 1750. He has beaten Enzo Ferrari in the new 8C2300!
And these are the pictures I had somewhere in my files...
Here is Enzo with the new 8C
It seems that stater lists or detailed results are very hard to find, at least for me, but I'll keep my eyes open for more infos ...