I took a look at this a few years back, and came to the opinion that the lighter blue, with blue reference sheet was the earlier printing, on the basis that the quality of print in both the book and reference sheet was superior in the yellow/dark blue copy. It was a completely subjective assessment, as I could get not varify my supposition independently. The dark blue/yellow ref copy seems to be the more common one and tends to be presented in better condition when it appears. My supposition was a first print run light blue/blue ref sheet, followed by a second print with more deffinition/saturation on the cover and the yellow ref sheet and slighly more definition in the print quality once demand was established. Unless the light blue copy is a from a batch of fakes, which I doubt.

I also found that the ink saturation on the blue card was not good, and as a result it was hard to read some of the charts, whereas the ink was darker and the contrast between the ink and yellow card was much higher, resulting in an easier to read quick reference sheet. Still, an independent confirmation would have been nice, but I couldn't get one. The e-mail leads I found all seemed to be dead or ignored.