Apart from the Sanhedrin, seventy-one carries little developed symbolism; it is a prime the older systems mostly passed over, its significance borrowed from the seventy it exceeds by the presiding one. Modern numerology reduces it to eight and reads it as a number of authority and material mastery. The honest core of seventy-one is the composition of the Great Sanhedrin, seventy elders and one head, a structure designed so that judgement could always fall one way or the other.
The whole apparatus of digit-reduction numerology is a twentieth-century invention rather than an inheritance from antiquity. Mrs. L. Dow Balliett published the first systematic version, The Philosophy of Numbers, in 1908, fixing the letter-to-number chart still used today and ruling that eleven and twenty-two must stand as master numbers rather than reduce further; her colleague, the physician Julia Seton, is credited with coining the word numerology itself around 1912.
A separate and older-claimed system, generally called Chaldean numerology, assigns letters to digits irregularly rather than in Balliett's repeating one-to-nine sequence, and deliberately omits nine from the chart as too sacred to attach to an ordinary letter; its popular billing as a genuinely ancient Babylonian system rests on assertion rather than on any textual evidence that has been traced back that far.