The S-42, was basically the S-18. Just with an assortment of modifications. They basically added in new fangled gadgets that they were... test bedding on them, before induction into the newer, soon to come off the assembly line, of subs. That, plus some minor tweaker body modifications to eak out a bit more service life outta the early on models... namely, the conning tower.
In short, consider the S-42, the S-18, with just a few more bells, whistles & fancy lights added into the new car package... just a lil' less... rusty, than their S-18 earlier on brethren.
M. M.