The use of a single lock bolt on American rifles is most common on southern rifles. In Kentucky, where I collect and research, it is common to find single bolt flintlocks after about 1825... I think 1820 might be a little early for single bolt flintlocks, but there is always an exception to every rule. Keep in mind that some late flintlocks made in the percussion era also had a single bolt similar to their percussion counterparts, Tennessee mountain rifles being a good example, and often get dated a little earlier than they really are. Shelby Gallien