251 is a C118A which was the military designation for the DC6A. It was delivered to the USAF in 1954. The DC6A has a forward cargo door and was designed from the outset as a freighter, DC6Bs on the other hand have lower floor loading and no cargo doors either forward or aft. All of our DC6Bs that we fly were modified with a rear cargo door.
Good capture, I usually don't even try when the lighting is like this cause I can never make the picture turn out.