Ok, so I've come across a reference to the six frigates owned by Prince Eugene of Savoy which offered fire support from the Danube. I've even got a picture of similar, if later built, Austro-Hungarian riverine frigates:
However, Drax, my expert source, says the only books with any detail are in Hungarian, Italian or German, none of which he reads.
Now this is a longshot, because the books talking about information were referenced in 19th century works... but having one reader who takes an interest in the Savoys, and one Hungarian, I thought I'd ask if you have any idea how early this concept of a two master with sweeps [big oars] goes back, the interest being, of course, for Jurko's navy.
