So I went digging by reading one of those reference posts further and found this:
https://www.mobileread.com/forums/sho...5&postcount=88
That member states that "DP75SDI" could be found in the "manual in the box"
That was a PW2 from Brazil, so it must represent some kind of alternative Amazon model designation which is different from the "nicknames" used here
Also did a search on ebay for "DP75SDI" and it showed 37 listings, all referring to Paperwhites, using that text either in the title of the item or within the body of the listing
It also showed up here:
https://wikidevi.com/wiki/Amazon_Kin..._WiFi_(DP75SDI)
Probably much ado about nothing, but keep it in mind in case that "model" number is mentioned again or one similar to it