What a remarkable calculator. I thought Commodore started with the Commodore 64 kiddies in the early 80's but I was surprised they go back to 1958.
That calculator is super fast for its time because it is all hardware based. The many calculators appearing in the 70's, like my scientific Farad-808 (I bought it in 1976, but its TI CPU goes back to around 1974, and the calculator is still used it on my desk today) used machine language firmware to make calculations and so was noticeably much slower a lot more more powerful. The method of doing the root of a number in those calculators was pure genius.
The multiplication bug could be pretty easily debugged if the author got a schematic, tracing out the circuit would take a lot more work. I would put my hand up to debug it for free if I got a schematic.
The HD3112 chip details are here. Quite advanced for 1969...
http://madrona.ca/e/eec/ics/serialadders.html