Whether section 8 of HSA 1956 or Uncodified Hindu law applic

Grand father got properties through partition with brothers in 1920. (After death of his father).Later he formed Hindu undivided family with his three sons. Grand father acquired many properties during his lifetime. He died intestate in 1952 and all his grandsons born prior to 1956. My question is does grandsons got right by birth according to customary law or grandsons will get right after death of their father according to section 8 of. HSA 1956. As it was held and reaffirmed in Supreme Court judgements that HSA 1956 is prospective not retrospective. Please who are expertise and experienced in Hindu law answer it. Others leave it for godsake. Response is highly appreciated.