A number of survey reports contain highly important dates. Some are from dates of purchase or putting into service. I then interpolated and smoothed the curve to fit the credible dates, and produced Table 2 with batch-production dates. The questionnaire inevitably sought too much detail on some things and not enough on others. However, it established a sequence of changes and patterns nicely related to the serial numbers.
Small dials have short hubs and shallow-etched figures.
Crystal filter shafts are positioned by a strip screwed to the front panel. There are other details which changed later, but they seem to me too trivial to mention. The first known change was to the S-meter with raised rim. The pin can easily be centered in the fork, preventing contact. Coil charts printed black on white are replaced by the reverse, with identical graphics.