Several people have identified the "Mystery Car" as an ex-Indy Ferrari 375 driven by Johnny Mauro.  Thank you to all responders!
 

The new mystery car is a 375 Ferrari 4.5 Litre Formula 1 modified.
Certainly one of the four entered at the 1952 Indy 500:  One entered
by the Scuderia for Alberto Ascari (DNF).   The 3 other are American
entries:  Kennedy Tank (driver: Johnny Mauro), Howard Keck Special
(Bobby Ball and Bill Vukovich),  Grant Piston Special (Danny Oakes).

These three did not qualify.

The top of the bonnet of the Ascari car was cut for the race after the
mounting "in extremis" of a battery of three 40 IF4C Weber carburetors
in place of the normal 46 DCF3.

The mystery car could be this one (photo).  Why the name of the driver
on the flank of the car is Johnnie Parsons, who took the start of the race
at the wheel of a Kurtis-Offy, is another mystery for me.

Salaam
 

Here are more responses, these from the "Racing History" newsgroup.
 

Hi Tam,

The picture Mr. Assari sent you is the Grant Piston Ring car,
Chassis No. 2.

The man in the dark suit is Gerry Grant of Grant Piston Ring.
The other man with the fedora is Grant's partner, John Bartlett.
Because of the crowd, I would reckon that the pic was taken
during the first weekend of qualifying because Johnnie (note
spelling) Parsons is in the car. During the second weekend,
Parsons  bailed.

He drove the Jim Robbins Offy for Frankie del Roy and
finished 10th in the race.  The Grant Piston Ring Ferrari did
not qualify.

Michael T. Lynch
 
 

Hi Everyone!

I am new to this site so I hope this works.  The blue mystery car is the
Johnny Mauro 4.5 liter, V12 Ferrari built in 1952 for Indy. Ascari qualified
a similar car after air scoop and carb modifications and ran in the 500,
dropping out early with a failed rear wheel bearing and wheel.

Johnnie Parsons was unable to get another one of the Ferraris (Grant Piston
Ring Spl) to speed.  I don't remember if the Mauro car was a third car or it
may have been the Grant car.

Mauro tried to qualify the Ferrari for the 100 miler held at Denver later in
52 and rolled it into the infield.  He subsequently ran it at Pikes Peak for
several years.  Don't know what ultimately happened to the car.  Hope this
helps.  I saw the Denver race which was won by Bill Vukovich in the
Agajanian 98.

Kent Burkhead
 

My guess was that this was Mauro in the Ferrari, but I thought I
would try the Ferrari GP experts over at Atlas F1 to make sure.
Shortly after the photo was posted, Michael Ferner came back with the
following information on the car pictured.

(The photo was taken) September 6, 1954, Pikes Peak hill climb,
Championship car class, round 8 of the AAA National Championship, car
#33, Johnny Mauro, Mauro-Ferrari, finished 17th of 18 starters,
16'12.0". He drove the car also there at September 1, 1952, finishing
10th out of 20 starters, 16'29.15". It was one of the four Tipo 375s
brought to the USofA in 1952, to compete at Indianapolis. One was
entered by Scuderia Ferrari, the three others sold, to Howard Keck,
Gerry Grant (I believe chassis #5) and Johnny Mauro. None of them
qualified.

Barry Lake
 
 

Bonjour from Paris!

An interesting information about our Mystery Ferrari (by Mr. Phil Hill in
the Ferrari North-America site)

    "I was Kirk Douglas the first time I drove a Ferrari Grand Prix car. We
were filming the final scenes for the movie, The Racers, and I was
standing-in for Douglas by driving a 1951 4.5 liter Ferrari that had once
run in the Indianapolis 500 as the Grant Piston Ring Special. I recall
thinking what a brute the car was, and how you¹d have to race it on a huge
circuit to ever appreciate that it was the type of Ferrari that gave the
company its first Grand Prix victory against the all-conquering Alfa Romeo
Alfettas".

Cordialement

Redouane ASSARI