I am assuming that the black-bottom car is the silver '99 Cobra (or some other late-model SN95 Mustang). If this is the case, then the factory goofed. Those were run at the same time as that year's club car, which was the poorly received '01 Mustang Cobra. That car also had a black-painted base with detailed exhaust. The fact that it was #1 in the series is pure coincidence.
If it's some other car, then I have no idea.
Some later series throughout 2003 and into 2004 had black-painted bases, a practice that has stopped lately.
Hope this helps...
jk