it is a Field's patent falling block, no 1927, from 1877
William Field sold about 2500 of his patented actions to various gunmakers, including Rigby, Bland, Holland & holland etc They were popular in the 1880s and 1890s

i don't bave a copy of Jonathan kirton's book on British single shots, but it is likely shown there.

This is a mystery, but there is likely a simple answer, perhaps it describes the condition of the old action

thanks

don