I have two sets of magnesium American Racing Wheels which were previously ran on Robert Christiansen's '69 Z28 in TransAm and IMSA GT races. One set are the 5 spokes (one of which has a cracked spoke), and the other set are the AR 6 spokes, in great shape. Both sets of wheels are 15 x 8. The 6 spokes have 4 inch backspace, and the 5 spokes have 4-3/4 backspace. Robert (Bob) who ran a gray/black '69 Z28 under the R.A.C.E banner for *(Robert A Christiansen Enterprises) ran the 'dry weather tires on the 5 spokes and rain tires on the 6 spokes...
My understanding is the when Trans Am began, AR had the 5 spokes, but esveral teams experienced cracking. AR began working on a stronger wheel, ultimately coming out with the 6 spokes, but by the time they were introduced, Minilite had gathered most of the market! The six spokes are considerably heavier than the 5 spokes, and are visably stronger.
I've got good front and back photos of both sheels, and Somewhere I've got an ad ran by AR for the TA70 wheels, if anyone is intersted in seeing them.