I think you are underselling the tremendous descriptive skill physicians of the past had compared to today. If you read case reports from 50 years ago, and before the level of detail and precision is rarely captured today. One only has to read Kraepelin, Bleuler, Jaspers and Schneider to see why they still have an oft overlooked influence on descriptive psychiatry today. In terms of psychological phenomena Freud obviously popularised the use of 'trauma' to describe a deep psychical wound, and Janet was interested in dissociation and its role in psychopathology. Even non-psychiatrically trained physicians were far better than many psychiatrists today at describing psychopathology. In the military much psychiatry fell to the neurologists and they did an excellent job. Medicine, and psychiatry as a branch of medicine, typically develops the most during wartime, and European psychiatry certainly made enormous strides during the great war, as did American Psychiatry during WWII such that psychiatry was not an unpopular specialty during the 1940s, partly due to the successes of treating soldiers with hypnosis and brief analytic therapies. As a side, such was the demand for psychiatrists during WWII it was possible for physicians to complete a psychiatry residency in 90 days!
My point is to suggest descriptive or diagnostic psychiatry was in its infancy in the early 20th century is not true. We have not really advanced in the right direction since that times (most of the advances have been made in basic science and therapeutics) and if anything psychiatry has been regressing with the vast proliferation of ill-conceived diagnoses that seek to confirm there is a psychopathology of everyday life. So if we were to examine military records (and this has been done) they would be fairly reliable indicator of what symptoms were being experienced. Flashbacks (archetypal of PTSD even if not necessary) was not commonly experienced. Shellshock etc is much more like conversion disorder or somatization disorders of today than it is PTSD as many psychiatrists have highlighted.
I do not for one moment want to underplay the deep psychological consequences of atrocious experienced lived through, in fact one of the reasons PTSD irks me is by wedding PTSD to trauma, we tend not attribute the importance of traumatic events in the etiology of other mental illnesses like schizophrenia, bipolar, depression etc. Acknowledging that people suffer because bad things happen to them however is not mutually exclusive from recognizing the complex context into which PTSD was invented. It made its way into the psychiatric nomenclature at a time when there was an extremely unpopular war, when the women's movement was highligting the tyranny women suffered at the hands of men and that the real battlefield for women was their own homes, when the psychoanalysts were being expunged from psychiatry, and when the term 'neurosis', 'reaction' or anything that ostesibly suggested a theoretical basis to psychiatry or etiology of a diagnosis was being removed from the DSM.