Here is another pic of an S-18 class boat, which I recently discovered:
Despite low-contrast and even poorer (level with the deck) viewing angle, note plating contours indicating the conical fairing for the "trunk", configured exactly and in the same location as already observed on this boat (see below) - but with the exception that its top hatch is not flush/recessed but here apparently a domed, hemispheric bulge.
There was a question about both the year of the picture date and identity of the boat - which I believe the (on-line) source seemed to have mistakenly transposed, between "28" and "38". The boat is almost certainly S-28, for which we have other pics showing her in later years equipped with this trunk (though not with a domed cap, but apparently a flat one). Meantime, the fit of both the heavy, forward light-post and, behind it, the "Tom-Tom-on-a-Spider" hydrophone rig cannot be from as early as "'28" (1928), but instead must (and very plausibly would) date to circa-1938.
This would thus be the earliest-yet pic of S-28 showing the trunk - proving that its installation predated even her pre-WWII all-black repainting (though not necessarily by much more than this). And the domed hatch would be an early/initial design, later replaced by the flush-closing version.