OK folks compared to many of our learned members (and I mean that as a compliment) I know very little. However, I am very good at doing research and believe I have a fair dose of good old common sense. I continue to maintain an open mind on this subject but, by nature, am following up with my own research. I am not offended when I get shot down, so have at it. I take such occurrences as just another learning opportunity.
The owner is quite busy at the moment, so it might be some time before we get more photos and information.
To Mikes comment regarding round vs flat faced locks: There are examples of English flat faced locks dating to the second half of the 17th century. The Henry Crips lock dated 1647 would be one example. Granted, it is a dog lock but, going from memory - which can be a perilous move at my age - the plate bears some similarity to the plate on this piece. What am I missing here?
Intercepting sear: Could someone post a photo of, or a link to, a lock mechanism (Twigg or otherwise) that has an intercepting sear? Although the concept might be the same, intercepting sears I have found on the net have significant mechanical differences compared to this lock. What is the documentation regarding the first use of such an arrangement?
Spring/stirrup/bridle: When did the stirrup first come into use and what is the supporting documentation?
Reused barrels: This is the suggestion I have the greatest difficulty reconciling. The reuse of barrels is well known and I have pieces with reused barrels. However, I cannot fathom a maker reusing a barrel on a piece of this quality and leaving the original name on the barrel. It just does not make sense to me when it would be so easy and advantageous to scrub the barrel and add the later builders signature. I would also question the practice of leaving the earlier builder's Guild mark on the piece. Please provide examples of high-end pieces where that has happened. Pieces such as that posted by backsplash75 are irrelevant because they are plain utilitarian arms.
We can get into architecture and fittings when we have more to go on.