It's not an assault gun, right? It's thought of as a SP gun? Or what? /PK
I think they were just having a laugh! You know that gallic sense of humour, they probably looked at a normal St Chamond through a fug of gitanes smoke, shrugged, and dug out a 120mm gun...
Well, I think that they can have been serious, but as I hinted, not as a Tank or Assault Gun proper (then they probably would have used a howitzer), but simply as a SP gun, just for the sake of mobility.
From my understanding, this was just an experiment. It was possible thought as a self propelled artillery piece, but it was more than likely just a way to up the fire power of the St. Chammond.
The British 60pdr has recoil cylinders atop the barrel, and they are quite seperate and distinct from each other, whereas this gun has the cylinders beneath and in some kind of common sleeve. I think the problem is most photos of French field artillery are of the really ancient ones without recoil cylinders, and that's what we're used to seeing. I've not seen a photo of a late model 120mm gun, but the 1913 105mm has a fairly similar recoil cylinder to the St Chamond gun:
I found a book at the Library of Congress last year on post-World War French artillery developments by Col. Rimailho (of 75mm mle. 1897 and 155mm CTR mle. 1904 fame) from 1924 that explains this particular 120mm gun. It was a St. Chamond design based on the carriage and recoil system of their earlier Matériel de 155mm court mle. 1915 (model 1915 howitzer). The design called for three weapons: a 105mm field gun, a heavier 120mm field gun and a 155mm field howitzer which were designated respectively Canon de 105mm Long mle. 1923, Canon de 120mm Long mle. 1923 and Canon de 155mm Court mle. 1920. All of these were to have used completely interchangable carriages and recoil systems with identical vertical sliding wedge breeches. They seem to have been intended solely for horse draught (signs of the growing backlash against upstart motorization I suppose?) I also suppose that these guns were intended to be a polyvalent artillery system in order modernize and rationalize France's heterogenous field artillery park. Needless to say, these weapons developments of the early 1920's were halted probably for the usual reasons of cost as well as the fact that the French Army probably felt that the late war guns already in hand (ie. 155mm C17S and GPF etc...)were perfectly adequate for the conditions then prevailing. They were also no doubt opposed to the idea of turning in their 155mm guns for a less powerful 120mm. There is a photo around and about of the 120mm towed gun which was no doubt reproduced from Col. Rimailho's book (I only have a photocopy from the Rimailho book and a color drawing from a general history of weapons that utterly fails to describe or explain this gun, an explanation that took me almost twenty-five years to find, as this weapon did not show up in any of the usual references). These guns were purely experimental; however the 155mm mle. 1920 experimental howitzer greatly resembles the earlier mle. 1915 which was bulit to the tune of some 290 pieces or so, if I remember correctly. These are fascinating glimpses of weapons developments in the inter-war period, especially the 1920's (so unwarlike a decade it was, or so we thought!), a period I find utterly fascinating.