The "coffin" tank car is a former N-C-O tank car. I don't believe that Swayne owned this car at any time; it is not shown (or listed) in the book on the Swayne Lumber Co.
For pictures of the car on the N-C-O see the N-C-O section on the Slim Rails web site. See figures 10 & 11 on the page linked below.
250 and 251 are not N-C-O numbers. The N-C-O flat cars were numbered 1 through 97. Maybe these are later SP numbers? If these are SP numbers they have to traced back through the 1946 renumbering and then we can probably identify the N-C-O numbers.