A very belated reply to this old thread, but I've just acquired a Tolley resembling your description but bored-out to 20g and fitted with a concave replacement rib. Still a lovely gun, and a credit to its maker, however. Serial is 5980. The address on the barrels is 1 Conduit Street, so address records suggest 1885-1894, but the original BP proof-marks fit the 1855-1875 rubric.