Assuming you are asking about smoothbore NW trade guns, and not trade rifles.......
Why not go with a North Star West? They are GOOD copies of the Barnett guns which were the most widely traded NW trade guns during the RMFT era up through 1840.
None of the American makers of smoothbore NW guns sold much to the RMFT suppliers until after 1840 and the Leman is a real late comer to the RMFT. Leman did not go into business until 1834, had no contracts with anyone until 1837 (a Gov't contract), and did not have a contract to the western fur trade until 1841....
Did some Lemans show up west prior to 1840, possibly, but so far no documenatation if that is important to you.