I have never seen a Birmingham export lock with a detachable pan and I've probably looked at several hundred of them, if not a lot more, in the last 40 years.
I do think that many "converted" guns may have been made as percussion using left over flint locks. I've had at least two of them — fowlers that had locks that were clearly made as flint but located to far back to ever have been used that way. One of these was also a club butt with short land pattern furniture and barrel so, at a glance it looked like a Revolutionary War musket... except for it's Ashmore lock and drum located at the very end of the barrel. Regardless of what it looked like, it has always been a percussion gun. Nearly all of the original guns I've taken apart have breech plugs that are much thinner than modern makers consider safe, sometimes only engaging by two or three coarse threads.
The Ruggles brothers patented their simple percussion gun lock in 1826 or 27... something that would have made no sense at all were not caps readily available at the same time. They also were not in Boston or one of the populous seacoast areas but in a small town in central Massachusetts. I've been there, it has hardly changed since 1827.