I have been studying locks and the arms they go with for many years, and just when I think I have figured it out, exceptions pop up. Many early German rifles, fowlers and pistols have round faced locks and round pans with a wide skirt. I was thinking that later German locks were flat faced with an angled pan - maybe most are, but now I see exceptions. Europe was a big place with much variety by location, gunmakers moved from Europe to England taking design ideas with them, etc. There is not a straight line evolution in lock design from all sources.
We can learn a lot by surfing the auction houses with English and continental arms, and studying their locks - especially when dated. Build a collection of photos and look for what is common, what is different. Develop your own understanding, but watch out for the exceptions.
You can always post a lock here for reaction from the group. There is no single reference to answer this question. Bob