Speedmaster - Serial number and Case Back

I was looking at a complete on 70/71 Speedmaster and was curious why would the straight line case back have a lower serial number then the 220 when the 220 has a Hippocampus case back?

From MWO "220 bezel was fitted only to watches produced in the second half of 1970"

Yes, in the book, but the authors updated the information confirming that the bezel was seen on serials up to 31.008.x and produced INTO January 1971. Which one of them also states here. That is serials from.
30.589.xxx to 30.599.999
30.990.000 to 31.008.xxx
The straight writing casebacks is correct on serials from 31.000.x and a few seen before that with serial 28.426.x
So most of them was produced after the ones with pre-moon casebacks and 220 bezel's, but yes a few early ones was overlapping, which is very common with Omega's in transition periods ;-)