covers lead to plenty of information. The postcard stands alone: to see
a 550x352 view (23K) click here or
on the thumbnail; to see a Big one at 792x490 (67K) click here;
to see a GIANT one at 1636x1012 (364K) click here.
In one of the big views you may be able to identify some of the cars or spectators.
Based on car order and arrangement and the flagmen's attitude I don't see this
as a competitive lap, more like parade or warmup. Shadows show the sun is far
enough south for October-November, too late in the day for practice. Any help?