The formula for local speed of sound is
LSS = 38.94 √(273 + 15)
which is 660.83
Formula is better
LSS at ISA SL = 38.94 √(273.15 + 15) = 661.06 Knots
ICAO atmospheric tables have the value 661 Knots.
NASA in one of thier websites has 661.7 Knots
My question remains what were they trying to evaluate? And why were they adamant that the value will end in .7