This approach might be appropriate for military arms, which were (more or less) made to a pattern.
There are a number of good musket books. For US flint, consider US Military Flintlock Muskets, the early years by Peter A Schmidt
We copied French muskets from the beginning. There is a new book out French Military Small Arms, Vol I Flintlock Longarms, Didier Bianchi, in paperback. This book is good, though it does not compare in the precision, if you will, of Schmidt's study. I believe it is the best available in English, maybe because it is the ONLY available English language book on French muskets. Well, someone will correct me mentioning some older book (like Bianchi's pictures better)
Sporting arms, the long rifles discussed on this site, used a wide variety of locks. Ethnic German makers in Pennsylvania often bought their locks from Germany, or make their own in Germanic styles.
Riflesmiths in those states with close commercial ties to England, e.g. New England and the American South, tended to use English made locks.
Yes, the earlier locks tended to have a concave bottom edge to the plate, "banana shaped" as the Brown Bess guys would say.
Relating lock styles to age is a rough way to go. Unless the gunsmith made his own locks, which some did, they used whatever lock was available. In the last century many flintlock rifles converted to percussion have been reconverted using a whole replacement lock. Some of the work is hard to tell, others obvious.
If you are new to this whole business, BUY BOOKS. For the best photos along with excellent history I would suggest Thoughts on the Kentucky Rifle in its Golden Age, Joe Kindig, Jr. 1960. Available from Shumway in York, Pennsylvania. Or used from abebooks.com Well, that is my personal opinion. Others on this site think the newer books are just wonderful. I am inclined to respect Kindig and Kauffman more than some.