"Here is my guess:  Mansfield, LA, March 8-9, 1958.  Jim Hall drove a 250S #69 to a win in race 4, but started race 7, the feature, with another 250S [race number unknown, but #68 is close to his other car]. In the feature he finished 3rd behind Frank Davis and Ebb Rose [who drove a 300S but normally used #44]."

"At Galveston in April 1958 Jim Hall raced one of his 250S Masers with #68 as well. However, there the shape of the race numbers was different and the photos are clearly not Galveston."

From German Maserati expert Walter Bäumer:

"Yes, this could also be the case!  It's then a 250S as the driver is quite tall in this smaller car than a 300S!   If so, its then either chassis #2431 or #2432!  Mit besten Grüssen / Best wishes!"

(12-23-12)   From Willem Oosthoek:

"It took me an extra five years and write a few more books, but I finally have the answer. The photos were taken at Cocoa-Titusville, Florida, on May 28-29, 1960. The spinning #68 Maserati 200SI is chassis 2427 with Ron Courtney from Miami at the wheel. It was his first race in the car, last raced by Dan Gurney in the 1960 Cuban Grand Prix. The #88 D-type shows Bob Kingham's chassis 516, which won all major races that weekend."