Farnham's diary notes his purchase of his rifle. He describes his testing of it, a problem he found, and the type of sight mechanism it has. He notes what appears to be its serial number.
I just read recently that civil war sharpshooters often purchased their own rifles with the expectation that they would be reimbursed by the Union government.
As printed covering some 73 pages, Farnham's diary contains much detail.