The switch to the box coupler with the simulated knuckle and lift pin is 1940. The shift to blackened journals is not quite so distinct but is also shown as "circa" 1940. The long activation tab is most certainly 1940 as it was revised in 1941. The trucks on the freight cars vary in height as they had to be made to adjust to the high and low trucks of the 600 and 800 series cars. The trucks per my source material, are referred to as Type IV-G and came both manual and automatic. There were 4 types of trucks IV-G m low and IV-Gm high for the manual trucks and likewise IV-Gs low and IV-Gs high for the solenoid activated couplers. The low versions have a 1/8"drop in the shank whereas the high versions have a 1/4" rise in the shank. The high versions were used with the last 2800 series cars with deep dish frames. In summary the 1940 box coupler has the simulated lift pin and a long vertical plate at the rear of the box.
There were numerous changes made in the 1940-41 era as the trucks had to accommodate both variations in the frame and (when they were solenoid operated) the length of the solenoid operating pin. However in 1941 in order for the trucks to "nestle" below the frame, the simulated lift pin was eliminated and car frame openings were enlarged. The 1941 coupler has no pin and a very short plate or in some instances no plate at all.
My reference material is ...."Greenbergs Guide to Lionel Trains, 1901-1942, Volume II" 1988 Edited by Christian F. Rohlfing and "Greenbergs Guide to Lionel Trains, 0 Gauge, Volume 3: 1937-1942" by Bruce C. Greenberg, PhD. 2024.
Hope this helps
Don
