Across both references, there are four known dial types categorized as Mark 1, Mark 1.5,
link Mark 1.75, and Mark 2. This odd nomenclature happens a lot with vintage Rolex, I would imagine at some point there were only two dial variants, and then someone found a slight difference creating Mark 1.5 in the middle. By the
link same process, the
link Mark 1.75 was "discovered" later down the collecting line.