Must be stuck. If I turn the little tab in the middle to the right, the entire center section moves together, the window and the ASA numbers traveling along in harmony, forever bonded at ASA 400. If you spin the tab to the left, it does change the film type.
So it's either stuck or I don't have the secret handshake.
It's a Rolleiflex 3.5E, late 1950s - the first model, I believe, that offered the built-in light meter. Oh well...
Everything else works perfectly, after a CLA from Advance Camera in Beaverton, Oregon. They are awesome! Gary