The drop gons went through two rebuilds. The 1918 rebuild was minor compared to the 1925 rebuild. 1925 is when the winding mechanism was changed to that the doors could be opened and closed from each of the four ends. Previously all the doors on one side were linked. 1925 is when the coupler pockets were changed from wood to metal and the metal framing was added.
A 1958 or 59 issue of the Rocky Mountain Region of the NMRA newsletter "The Helper" had a history of the cars including the rebuilds along with some beautiful drawings by Billi Bowen.
This issue provided the info for the Grandt kit.