The boxes in the shoe shop are clearly all empty of boots.
Pvt. Walker appears to be using a black marker pen when drawing cartoon faces on the others feet. These were not available until many years after the war ended in 1945.
It is difficult to believe that Captain Mainwaring would not recognize a brand-new pair of boots from one that he had been marching in for a long period of time.
During Captain Mainwaring's initial man-by-man boot inspection parade, he totally bypasses Frazer to move along to Jones (where the next gag is lined up).