Yes you are correct, in addition to the adjustment knob the other difference is a metal heat exchanger. If you are going to dive in colder waters <10 then the 40 or 50 would be a better option due to heat exchanger. The 40 is identical, save cosmetic diffs and adjustment knob, to the 50, 100 and 200. There is a guy on eBay selling his own adjustment knobs at a cheaper price but they do not have the micro adjuster as per original factory part. I converted some ATX40 to ATX50 using factory parts and it was expensive. If you think you want the adjustment and heat exchanger you'd likely be better off buying the 50 off the bat.