There’s just one photo of a 3CLY Sherman in there, AFAIK, and that’s of the one exiting the Landing Ship, Tank on Sicily. That photo is why I’m wondering about the mystery item in the first place:
for a model of that particular tank 
When I began wondering what that thing is, I started noticing it in photos of tanks in North Africa as well, and that’s why I included a picture of one of the El Alamein Shermans: to show that it wasn’t a thing specific to the Sicily landings.