In answer to the question
if [what is depicted in linked video] was really possible in 1940?
The answer is unequivocally YES - in fact quite a bit earlier. Reanimating a severed head was hypothesised in 1812, first attempted in 1857 and, for the most part, perfected in 1928-29.
In 1812, French Physiologist Julian Jean Cesar Legallois hypothesised that a head could theoretically be kept alive in isolation from its body by maintaining a supply of blood. However this hypothesis was not tested until 1857 when Dr Charles-Édouard Brown-Séquard lopped off the head of a dog, drained the blood and after 10 minutes injected fresh blood back into the arteries. He reported signs of life displaying what appeared to be voluntary movements of the eyes and face. This continued for a few minutes until the head once again "died".
Source: Brown-Séquard, E (1858) L'encephale, apres avoir complement perdu ses fonctions et ses proprietes vitales peut les recouvrer sous l'influence de sand change d'oxygene. Jounal de la physiologie de l'homme et des animaux.
Similar research continued with Dr Jean-Baptiste Vincent Laborde who was first to fill a severed human head with blood. The results were disappointing, nothing much happened. Laborder blamed this on the delay between getting the head from the gallows (The subject was reported to be a murderer named "Campi") to his labratory.
Laborde subsequently tried again, this time on the murderer "Gagny" whose head he received a mere 7 minutes post-mortem. By 18 minutes he had connected the carotid artery to that of a still-living dog. Laborde reported that the facial muscles contracted, while the jaw snapped violently shut. No signs of conciousness were reported.
Around the same time, Paul Loye (A collegue of Laborde) erected a guillotine in his office and used it to decaptitate hundreds of dogs in order to study their reaction
After Laborde and Loye, a handful of doctors pursued this line of research but for real breakthroughs the world needed to wait until the late 1920s when Soviet Physician Sergei Brukhonenko succeeded in keeping the isolated head of a dog alive for three hours. What made this possible was the use of anti-coagulant drugs and a primative heart-lung machine (developed by Brukhonenko) which he called an autojector.
Source: Brukhonenko, S (1929). Experiences avec la tete isolee du chien II: Resultats des experiences. Journal de physiologie et de pathologie generale.
Which leads us back to the question. In 1940 a film was released detailing the work of Dr S.S Brukhonenko and entitled "Experiments in the Revival of Organisms". Dr Brukhonenko is credited with Writing and Technical Direction roles.
A remaining question is whether the video in question is an accurate depiction of the very real scientific work of Brukhonenko et al. The answer to that question, I am still looking for references!
Much of this answer has been researched with help of the (rather fun) book: Elephants on Acid (ISBN:9781743291870)