When units are operating in multiple and the gangway is in use, the gangway door and flyscreen acts to block access to the secondman's side of the cab. On Cl317's, the door itself only reaches to the end of the secondman's desk so the flyscreen is needed to block access. I would imagine that Cl153's have hinged flyscreens at one end because the cab is much smaller, which would require it to be folded in half.
However, I can't be definite about this as I've only ever driven a Cl153 briefly on a low adhesion driving course last autumn, and only then from the large cab.
one TN