Ron,
Thanks for the kind welcome. And thanks for adding the correct formatting. Although I've been collecting for quite a while, this is the very first time I've ever actually submitted pictures of any of my pieces for review by the world :-)
In regards to the lock, it has definitely had a frizzen, frizzen spring and a pan removed. One can clearly see where the remaining tell-tale attachment holes have been plugged. Whatever was used to fill those holes now has taken on an aged yellow coloured patina. Just as telling is that the lock is secured by only one side bolt although the plate, as a flintlock, was originally supposed to utilize two. What appears to be a bolt in the front is now just a very old filler. If the gun had ever been a flintlock (and assuming this is indeed the original plate as I am very sure it is) then it would probably have been, and would still be secured by the usual two bolts. Another sign that this piece was never a flintlock is the fact that there doesn't appear to be any tell-tale indendation in the wood near the tang behind the hammer.
It's my understanding that the pre-build modification and utilization of older flintlock parts was not entirely uncommon during the percussion era. Samuel Augustine was active between about 1838 through the 1870s. This would appear to be one of his earlier pieces (1840s) so I think this is a case where he used what was readily available.
That's my best analysis on the plate conversion thus far. I would sure welcome other comments and opinions from those who may be far more knowledgable than I on the subject.
Thanks again and have agreat day,
lboyle