Curious observation about serial numbers on Gold Rolex 1601/8

[bradleyt09]

I recently purchased a beautiful 18k Gold Datejust, 1601/8. The serial number is 3.9M corresponding to a production date of approximately 1975. The watch has a lovely sigma pie-pan dial, which is correct for this manufacture date. I have been extensively researching the 1601/8 on the internet to learn as much as I can about this watch. It appears that there are many gold 1601’s from the ‘60’s and early ‘70’s, and many from the late ‘70’s (5M) serial number. I have not found a single reference that was manufactured in the mid-70’s, or with a 3.9M serial number.

Does anyone have a 1601/8 from 1975?
I’d appreciate any information about the apparent cluster of gold 1601’s from the early and late ‘70’s. Also, any information about this model, in general, would be appreciated. I’m assuming production numbers were far less for this model than the stainless and two-tone versions, and believe there were also many more 1803’s produced.
When this watch was sold new, was there a significant price difference compared to the 1803? If not, why would one buy a gold DJ instead of a DD?
Thanks for any feedback.