Imperial Chinese copper coins had a square hole in the middle precisely so they could be strung on cords. A standard 'string' was nominally 1,000 coins (
一贯 yī guàn or
一吊 yī diào), and
钱串 is the physical object — the bundled string of coins. This is why
串 (chuàn, a string/skewer) became a counting unit for cash.