Let p be a rational prime
≡ 1 (mod7). Williams shows that a certain triple of a Diophantine system of
quadratic equations has exactly six nontrivial solutions. We obtain here a congruence
condition which uniquely fixes one of these six solutions. Further if 2 is not a seventh
power residue (modp) then we obtain a congruence (modp) for 2(p−1)∕7 in terms
of the above uniquely fixed solution.