Your gun was made at a time when the only powder available was black.It follows that the origional proof was for black powder. The 850 bar proof mark is for a Nitro powder test, applicable from 1994 to the present date. As such it is classified as a reproof.
In the Birmingham rules of proof applicable from 1954- 1994 reproof marks included the letter "R" surmounted by a crown. I am not certain as to whether or not the Birmingham proof house continues to use this mark when a gun is reproof tested from black to nitro. It could be that the mark is reserved for guns that are retested because of barrel wear/ enlagement[ ie; For a 12G.having .729 origional bore size,retested at .740 due to wear].
