haven't really looked at the twist rate, as you state, this can be a variable. Instead, the resouce I pointed to states that 12 grooves are at the extreme end of the number of grooves normally seen in military rifles. This same source states that this 12 groove rifling as something seen from Wurttemberg. Which just happens to be the same region that Staudenmayer was born in.
Staudenmayer, during his career (1799-1824) in London, was well known as the only real expert in rifling. Which says to me that he brought his rifling skills from his home... Wurttemberg.