Found this on the net today. A friend in the UK checked with Helston's for me about some of the details of this lock.
The 4 3/16" plate length measurement is accurate. Tumbler does not have a fly. My guess is that it was originally for a Officers pistol. I have never seen a British rifle with out a fly in the tumbler.
http://www.helstongunsmiths.com/default.asp?ID=995&CB=15%2F01%2F2015+12%3A04%3A36Not sure of the actual markings inside the plate but the text of the ad implies "Joseph Brazier Ashes", if this is the case the lock would be the highest quality. Lesser quality locks are nearly identical in appearance but are marked "I.B." or "Jos. Brazier".
This would be a great start for someone desiring a high quality percussion lock. Not very usable as it is because of the plate size but making a new plate would be fairly simple along with 4 new bridal screws and a sear spring screw. Surely the existing screws are some obsolete 55 degree thread. Price at current exchange would be about $225 USD plus shipping.
Jim