Don't know the level of truth in this, but it could answer your question about the older Grp4 cars being even lighter:
"While a projected weight of around 1100kg had originally been targeted, Porsche were so concerned about the Pantera’s arrival that they forced the FIA into homologating the Group 4 version (or GT4) at 1250kg"
Source: http://gt-cars.co.uk/1974-de-tomaso-pantera-group-4/
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