Just as an aside, I probably had the same gun (hammer double with side lever) Mine was marked 12ga and 38-55 (probably for the American market. Upon checking the bore and chambers, the chambering was actually 9.3x57r/360. 38-55 would fit in the chamber although I never fired it.
Good luck and that might be the cartridge for my next DR or single shot build.
Tom
-------------------- Carbonation without fermentation is tyranny