With thanks to Dani Stopsel via Facebook whose post of a WW1 drawing by Marcel Santi led to my research
Marcel joined the French Army’s 2nd Infantry Regiment (42e régiment d’infanterie or 42e RI) during the First World War and was sent to the Western Front. Marcel was a veteran of the Battle of Verdun* and he sketched the lives of his fellow soldiers in the Trenches during WW1.
The 2nd Infantry Regiment (42e régiment d’infanterie or 42e RI) was an infantry regiment of the French Army, raised in 1635 as the 'Régiment de Calvisson'. It was renamed the 1638 'Régiment de Montpezat' in 1638 and the 'Régiment du Limousin' in 1684, before first gaining its numeral of 42 in 1791. During the First World War it was nicknamed the 'As de Carreau' (Ace of Diamonds), since it was part of the 'Division des As' (a nickname for the 14th Infantry Division). The Regiment was disbanded in 1991.
Sources:Original source - with thanks to Dani Stopsel via Racebook who posted this image by Marcel Santi in remembrance of Verdun
https://www.mutualart.com/Artist/Marcel-Santi/ADDA5FD39C909A19/Biography
http://www.kaiserscross.com/136501/home.html
https://forum.pages14-18.com/viewtopic.php?t=54720
https://www.maison-heinrich-heine.org/culture/2018/novembre/marcel-santi-1897-1986-carnets-de-balles-et-de-voyage-1643?lang=fr
https://en.wikipedia.org/wiki/42nd_Infantry_Regiment_(France)