I just found another pic, revealing another view of the small, sponson-like gun platform support noted earlier for S-30:
Click on Image to Enlarge
At lower left, the pic is annotated "S-30 - S-33 - S-20", which raises two possibilities: 1) the inboard sub is again S-30 - and, viewed from their bows and listed left-to-right, the group would indeed begin with this sub - or; 2) this is discovery of a second "mini-sponson" - identical to
S-30's - fitted to S-20 as well.
In any case, the big "news" about this pic is that it dates firmly to the '20s. The precise date is annotated (again, at lower left) on the pic: "Oct 2" of either "1923", "1926" or "1928" - I simply can't make out the last digit - not even with any amount of CG help. However the '20s time period is further confirmed by the dorsal skeg (under inset) on the sub in question (and no doubt the two others as well).
Either way, it confirms the earlier conclusion that these gun platforms were fully in place by (at least) the late '20s, and that the "mini-sponson" prototype remained virtually unchanged (at least on S-30), for the following two decades!
Kinda pathetically geeky to be getting worked up about this sorta thing, huh?