Washington DC Carrier Cover

Have this envelope mailed to Fred Brackett G.A.R he was a Major during the war. What I would like to know is -- I see it took 2 days to get to DC then 2 hrs. for carrier stamp what is the 1 hr differance to box department.? If it was received in DC how far could it have gone? or was that the time it was picked up.

As I understand it, carrier delivery was a special service in the 1870s and '80s; most mail was held at the PO for pick up. I'm not sure what the box department cancel indicates, but I'd like to know more. I also have a number of "carrier" backstamps on covers, primarily from DC. Do any other collectors know how the post office determined which pieces of mail were worthy of a carrier delivery? There doesn't seem to be an additional charge assessed.