Stu "Dr. Etceterini" Schaller, an expert on small-bore Italian sports/racers, identified the car as a Cisitalia.  A look at the Cisitalia website (Entrar-Galeria-Pasado-2-6) shows the "Cisitalia 202 SMM Spyder Nuvolari".  The details of that car and this one match up, except for one thing -- that pesky Maserati trident in this car's grille!

Here's Stu "Dr. Etceterini" Schaller on this car:

"I think the car might even be one particular car, called the Razzo, rather than a "standard" Nuvolari spider.   If you look close, you can see kind of wings at the top of the front wheel arches, and as far as I am aware, the Razzo was the only Cisitalia spider that had them.   The Razzo also had cut down doors, but it is impossible to see if the car has them in the photo shown.   Regardless, I am 100% certain that it is a Cisitalia; you can even see the Cisitalia badge between on the top of the nose of the car, in front of the hood.   The standard motor would have been 1100cc, but it is certainly possible a 1500 or 2 liter Maserati A6 motor could have been put in the car."

Earlier your webmaster mistakenly identified this car as perhaps a special-bodied Maserati A5GCS.  This error came about because the Stockton results (From Bob Norton's MotoRacing scans Vol. 1 "Results") show Chuck Tannlund finishing 6th o.a. and lst in class E Modified in his Maserati A6GCS.   There's no mention of a Cisitalia anywhere in the March 23, 1958 Stockton results.

If someone actually transplanted a rare, fragile, expensive and exotic Maserati A6GCS engine into a super-exotic one-off Cisitalia Spyder it would have been of extreme interest to enthusiasts in period:  IF THIS HAPPENED -- WHY HAVE WE NEVER HEARD ANYTHING ABOUT  IT?  

Stu & I started a discussion of this car on the Atlas F1 "Nostalgia Forum".