“harmonic balance” is more of a marketing ploy. The best way to find the COP is to hold the sword by the grip with one hand and whack the pommel. The part of the blade that doesn’t vibrate is the COP or “sweet spot”.
if my memory serves me its talhoffer. if he dosn't name the source after the quote than it belongs to the last source he mentioned. i had the same problem yesterday in the same book about a charles the bold quote.