i can hold the advance button down with the back off, the wheels move freely ..
i have a dummy roll of film i would in the camera with the back off and everything worked perfectly
i also thought the backs wouldn't matter, i was told ( and have read ) they interchange and all is good ..
but ... at least with mine, the plates seemed in different places so they applied pressure against the film
which in turn didn't let the spindles wind freely ...
thanks for your advice !