UNREP at all did not occur before the 1930s. From the 1930s through 1940s (unsure of exact stopping point), it only occurred while basically stationary between ships of any type - and, as noted, not for subs. Subs required dedicated tenders, which would usually be parked in a specific harbor (and well defended). The Germans had something of a special case exception to this during the brief period the Milchcows operated, but the Allies never did anything like that.
(Aside: the first warship -> sub UNREP was in 1943, between DD-818 USS New and SS-423 USS Torsk.)
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