I realised that my Chrono Master has a lug width of 20mm, which was relatively rare in the late 1960s when 18mm was very much the standard. After some further study of catalog images, I reckon that the models that were sold only on leather straps had 18mm lugs, whilst the ones which had a bracelet option had 20mm lugs.
I’ve now added this info to the Chrono Master page. It’s been ‘Amendment Day’ today! 🙂