On the 3000 cars without the "Camel" doors, there are quite a few differnt styles. Most have to do with the door hardward as you spotted. Some have two runners to hold the bottom of the door some have three. And as you noticed some have the tack board and some do not.
It seems the biggest difference in the 3000 is in the 1920's rebuild, roughly 60% of the 3000's got the new doors and door guild built into the side.
Maybe there is a data base out there that lists what cars got the rebuild and those that did not. I'd like to have that for modeling...
Kevin Cook