There are some trade records of their guns going to Kaskaskia traders in Garavglia and Worman's book and Charles Hanson notes a reference to the brothers having provided a pattern gun to AFC around 1830.
The actual references are......
1824 - Menard and Valle of Kaskaskia:
"2 Com[mon]rifles Dickert Gill @ $11.00 and 1 best D[itto]at $12.00"
1826 - J. J. Astor letter to John J. Henry:
"...to be the same as the sample Rifle of J. Dickert Gill now furnished by the Compy."
as noted above the most likely explanation for those marked Dickert Gill would have been those built by J. Dickert Gill and not by eitehr partnership. Those marked Dickert and Gill could be of the earlier era, but could just as likely (perhaps more so) have been one of the later ones by the J. Dickert and B. D. Gill shop.If they are marked Dickert Gill, that would indicate that one of them made the gun. The one I mentioned that was talked about in Kaufmann's book was marked Dickert & Gill. That indicates that Jacob Dickert and James Gill sold the gun through their store since James Gill didn't build rifles
Third option a parts gun with a Dickert and Gill barrel, Leman lock, etc. by an unknown maker - something like the often touted iron mounted "Henry" which has a Henry barrel and maybe lock, but is in fact a NC built rifle, albeit a signed one.I think the gun in question is late 1840's or early 50's. It also has a Leman lock and buttplate. The options there are that it was a Dickert Gill product using parts bought from Leman, who regularly sold locks, hardware and barrels to other makers. Its also possible that it is a Leman product with a barrel salvaged from another gun. If I were a betting man, I would say the former. Other than the parts, it doesn't really say percussion era Leman to me.