OK, thanks for updating us ... "I own the original yellow gold Seiko 66-1001 ... I can attest that mine is an original Seiko."
Does that mean you are the original owner & that you got it from some kind of "authorized dealer" or other "authorized" sales channel?
I have seen various kinds of "unique" vintage Seikos, in some cases they were company-created "specials", usually involving some personal engraving. I also saw one with a quite unique dial where the dial numerals were replaced by letters spelling out the owners name (though I do not know if Seiko ... meaning Hattori Corp. ... did the work or if he had gotten it done by some other means; it was being sold off as part of his estate by an eBay dealer here, so he would not know the history unfortunately).
I don't say it's impossible that Seiko / Hattori did make some kind of "bespoke" model in those days [which would account for the known population of 2 of these, one yellow and the other white]. But, I've heard of no other cases like this from the ti me period. Some kind of actual documentation of these would be quite helpful in establishing the provenance/history of yours (and the other white gold example). What can you say about their "backstory", why would a rather common, low/mid-grade movement of that era be used in a clearly "luxury, solid-gold, extremely limited production" model in that time?? Why not a better grade movement from Seiko if one were going to all the trouble/expense of having a "unique" bespoke item crafted for them?
But, the more I (and other collectors) study Seiko/Hattori in the "old days" the more we find that there are exceptions to the rule, and exceptions to the exceptions
I do think it is a very beautiful piece, and if indeed both examples are hand-wind ... as they should be, given the case/calibre number ... then it leads more credence to them being some kind of "legitimate" Seiko/Hattori product (even if Seiko/Hattori) went outside to have the cases made. And as I remember, each case shown did have some differing design details. That is unusual in itself when sharing the same case style # ... again, another puzzle.