I read that the rear exhaust covers and the large hatch were features added to the 1941 model, but in these two picture, and many others, really, you can clearly see that the features were the same on the M1940. So my question is, are all the sources I found wrong, or how else can this be explained?
IIRC, Soviet wartime production wasn't as concerned with consistency across all tanks as much as efficacy and sheer production numbers. There were many factories and workshops producing parts for T-34s; given the rushed nature of production, it is not at all unlikely that a Model 1940 tank got some Model 1941 parts, or vice versa, assuming the parts were compatible (which they should be, considering that upgrades were being developed incrementally on the same chassis).
In the image shown, it's entirely plausible that a 1940 hull was mounted with a 1941 turret, giving the appearance of a 1940 tank with 1941 features.
According to T-34: The Red Army's Legendary Medium Tank, by Anthony Tucker-Jones, the transmission access hatch was supposed to be rectangle shaped on the T-34-76A and some T-34-76B, and round on the rest of the T-34-76B and all subsequent variants. The cast exhaust pipe covers were the same shape on both variants. Wartime production forced factories to use whatever was available, however.
Keep in mind that there was no consistent policy of naming T-34 variants. Some western sources use German designations for the T-34. The T-34 Model 1941 in the first picture (with the mantlet "chin" and long F-34 gun) was called the T-34-76B by the Germans; the T-34 Model 1940 in the second picture had the short L-11 gun and was called T-34-76A. Some Soviet sources call the first T-34's Model 1939 instead of Model 1940.
When you mention the hatch, are you talking about the turret hatch, or the transmission cover hatch? If I remember correctly, the only modifications to the turret for the 1941 model were to support the new 76mm installation. On the other hand, the rear of the tank was modified on the 1941 model somewhat to change the configuration of the exhaust covers and to introduce a larger, circular transmission access hatch.
That being said, it's hard to identify "quintessential" models of Soviet tanks. Turrets were manufactured separately from hulls, so it's quite possible to find new turrets on old hulls and vice versa. Not only that, but different manufacturers made their own modifications to turrets and hulls to support their manufacturing methods. For example, the STZ factory in Stalingrad never introduced the circular transmission access hatch - instead all their tanks had rectangular hatches. Also remember that in late 1941 and early 1942, the Soviets were rapidly relocating their industry beyond the Ural mountains in to keep it out of reach of the rapidly advancing German army. As a result, there are lots of "chimeras" around that will leave you scratching your head.
