That was what I was thinking but didn't know if it were possible. In that case the 0644 likely was the number from the barrel maker.
As for the action the Jones U-L was patented on 07 Sep 1859 so it theoretically was possible that the action could have been built in late 1859 as an U-L and mated to the barrel flats (which might explain why they would be kept and a new barrel mounted). The earliest (confirmed by photos) extant Reilly with a Jones U-L in my database is SN 12543 (1862), a pin-fire 12 bore SxS shotgun.
Last edited by Argo44; 11/13/23 08:52 PM.
