I do understand your confusion.
Fact: BMAG used to produce a 57Kg torpedo for export, looks like that at least Sweeden, Italy and the US bought it (check the war head weight in the USN weapon's monograph).
Fact: B57 is defenitely not a Whitehead torpedo.
Fact: Rossler is a good historian, I own several of his books being me interested in submarine as much as I am in underwtaer technology.. but that paticular book is not the bible, it clearly lacks big pieces of the early period.
Fact: my german defenitely sucks.
Now, when the Royal ItalianNavy used to buy both Whitehead and Schwartzkopf products (do you know what Schwartzkopf means? black head, funny enought..) they changed the nomenclature using A for W. (that stand for "acciaio", steel) and B for S. (that stand for "bronzo", bronze).
The Royal Italian Navy agreed in buying BMAG products only if the company was willing to open a factory on the Italian territory, so, in 1887, a BMAG torpedo factory in Venice was biult; from that facility comed out around 650 B57 and around 85 B90, just for the Royal Italian Navy (so also in Berlin they used to produce the 57Kg variant).
Since the products couldnt really compete with Whitehead's one, with particular reference to the B90, the Royal Italian Navy closed its relationship with BMAG forcing the closure of the Venice factory in 1901.