Because its an earlier model year. The rib came about later on. According to the registry the transition started around Feb '72 with car #2750 or so. As with all things DeTomaso there is rarely a distinct time in the production where changes are incorporated. Car #2750 may have had the rib and car # 2751 may have been flat.
There is a functional element of stability of the lid as well as the flat lid was felt at the time to warp due to heat. Not sure if that is true, my PB has the flat one as well but I can't say that I have noted any issue.
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