Just when you think you have got all the answers as to what labels appear on what models, an exception comes along..
Here is an early Series 4 (1954-1968) with the more modern style non-parallel sided shaft, produced alongside the more traditional parallel sided shaft models (eg 73). This is a model 973 which makes it Right Hand thread (the 9), a 15 deg head angle (the 7) and 12" ( the 3). All these were thought to have had rear markings of AB BAHCO STOCKHOLM or BAHCO SWEDEN, but this one has the A/B BA HJORTH & CO STOCKHOLM which was only thought to be on Series 3. There appear to be "transition" models which help to date the artefact. This wrench may be one of the first production runs of the 9 series made around 1954.