Bob Ford was born and raised in Ray County, Missouri, 400 miles south of Walnut Grove, Minnesota.
Charles mentions the Civil War has been over for 11 years. So it is 1876.
When Charles swims from the mill and under the bridge he uses the freestyle stroke, arms over his head and legs in a scissor kick. That stroke had not evolved by then. It was introduced to the world in 1896 Olympics in Athens, Greece.
When B.K. Dankworth gives Mary a job offer to make deliveries to his place from town at 25 cents a day, Charles later mentions that her new job paid 10 cents a day. Correction: during the initial conversation about Mary's job, Charles says 10 cents a day is enough for running errands.
At the end Mary mentions that Bobby Ford killed Jesse James 6 yrs later in 1882. 1882 was when Jesse was killed, but that would make the current year 1876. 1876 would be the centennial which was a few years before.
At 28:00, Mary refers to the James brothers as "Dobbs and Dankworth" instead of their aliases "Hobbs and Dankworth."
It's completely out of character for Charles Ingalls to allow his 14-15 year old daughter to be alone in a house with two handsome male strangers given the Victorian social standards of 1876.