Screw type controls are listed in the 2a parts book with no changeover suffix, just with the reference "alternates", suggesting they were not confined to Series 2. My memory says they changed in Australia at least, very early in Series 2a production.
In any case it is inconceivable that the change in vent control would have coincided exactly with the change to 2a, which was defined by the introduction of the 2.25 diesel.
John

