British patent protection is 14 years. A gun with a 1867 patent mark would not need to be marked after 1881. Sometime between 1867 and 1881. That puts your guess square in the middle of the 1870s.
I think the key forend is actually what is known as a wedge forend. It was a hold over from muzzle loader design until other methods were invented.
A popular improvement for forend latching was the Deeley and Edge latch. Just pull down the latch on an early gun and look on the edge of the latch to see the marks.
Basic dating of English guns (and guns from the continent) follows 4 paths:
1. Published serial numbers or original records.
2. Name/Address, makers moved and merged alot. The address/name can put you in a date range.
3. Patent use marks. They put you within a 14 year range of manufacture.
4. Proof markings, rules of proof changed over the years. The date range is really varied. Most US owners dating an old gun are concerned with pre-1898 to determine antique. The British are concerned with pre-1896.
Posted photos help us ID guns. If you can post the watertable photos it would be the biggest help.
Joe
