I checked your link to the hunter site and randomly zoomed into some cover images and they all seem to be labeled HUNTC (not -D) for the ones I spot checked.
For the numbering, it's possible that the new JAV laws that require 3 month hold between filming and release screws up the numbering order vs the release order. Like a production will be assigned a number before filming, but then the delay ends up with higher numbers being released before lower numbers due to different filming or editing dates plus the 3 month wait period.
Here's what I've been looking at but now that I zoom in enough I realize I saw it wrong. For some reason these looked like D to me.