yes, i know my sh!t and all SKS-D was converted till they ran out. weird you judged a conversion from a singular component (trigger) rather that the main part, the receiver. all SKS-D receiver was converted from a regular SKS receiver part, only when they ran out did they make new ones. there are "D" out there with carrier milled for stripper clips but when that ran out, they started to make carrier specially for the "D" , same goes for the trigger and stocks.
interesting read in the link by horilka, go to the detachable D and midwest sections. midwest ordance started all this. i once had a collection of 4 "D".