I came across these two images at the US National Archives in one of the US Army Ordnance Bureau files. The caption identifies the cannon as a 6-Pdr Tank Gun Model 1920. Does anyone have more information about it?
Very little. I believe that the gun was a development of the naval 6 Pdr used in tanks during the Great War that had a redesigned breech block to make it easier to load in a confined space, effectively by turning the block through 90 degrees. Beyond that I can't help.