Thank you, gentlemen. I recall the much photographed KO'd specimen that every feldgrau with a Kodak must have taken a picture of, but wasn't sure what Mark it was. I am currently building the Scale Link MkIV, and in 1/32 scale brass & white metal, it is a whopper of a model.
I thought that grousers/track extenders were not used the first time the Mark I tanks were used, but were used the second time they were used along with the Mark II tanks (at Arras, as Centurion says).
There are quite a few photos of bashed up Mark II tanks with the grousers on: