speedometer cables

[BCA]

Ford F15A cables came in 2 lengths; about 72" and about 84". From the F15A parts book the 72" were for "old design cab to #21565" and 84" were for "new design cab from #21566". Does this mean that the transition from Cab 11/12 was at serial #21566" or does it mean that the 84" cable had the different fitting for the round military speedometer head? I don't see any reference to the round military gauges in the May 1943 F15A-03 so therefore there may not be any speedo cable reference for Cab 13 Fords with the later Round military speedometer. Any ideas when they switched from the civilian to military standard gauges? Ignoring part numbers how long does a speedo cable have to be to work in a Cab 13 Ford? ....... Brian