The 1620 was an odd duck: variable-word-length decimal, as you point out, but targeted at the scientific market -- there was even a variant, sold as the 1710, which added extra hardware for industrial controls. (The 1401 and its successors were the nearly-contemporary low-end business models; there are a bunch of differences, but perhaps the most significant is that a single 1401 memory location could hold a decimal or alphabetic character; the 1620's memory locations were digits, which had to be paired when representing characters, making coding for anything involving character data a whole lot more awkward.)