Greg,
Atkin, Grant & Lang is still in business. Here's their website
http://www.atkingrantandlang.co.uk/ . Ken Duglan may have the records of when your gun was built - he was able to find the record of my Joseph Lang which was built in 1866 including the dimensions and name of the orignal purchaser. I'm pretty sure that Ken will be at the Southern SxS in Sanford NC next weekend and will return to England shortly after. It's always great to look at his new and reconditioned Atkin, Grants and Langs and I will be be drooling next weekend.
