Bill,

I am currently working on two of these cars which Marc Simpson recently had re-run by Sylvan.  The prototype notes included with the instructions suggest the boxcars with wood doors were built in 1929, and those with steel doors in 1930-31.  It also mentions doors were sometimes swapped from steel to wood and vice versa during shoppings and repairs.

In Ted Culotta''s Steam Era Freight Cars Seminar Manual he has a photo of CN 503559, built in 1929 and it does have wood doors.  There is also a photo of CN 506951, built in 1930, with steel doors.  Ted references an article in Railmodel Journal, June 1994, by Stafford Swain.  You can view, but not print it, here:

http://magazine.trainlife.com/rmj_1994_6/

I also referred to Ian Cranstone's web site for information on these cars:  http://www.nakina.net/cn/cn5.html

I hope this helps!

Jim Brewer