I've always wondered why the Klingon ship is referred to as a D7 even though they used the DS9 K't'inga-class CGI model to represent it. Weren't there meetings between the writers and the SFX artists?
Although various technical books have drawn a distinction between the Klingon battle cruiser as it appeared in The Original Series
(D7) and The Motion Picture
and beyond (K't'inga), I've always thought they were the meant to be the same. After all, the only difference is the added detail that wasn't possible on TOS' budget in the 1960's - and much of that detail (hull plating, glowing engines and torpedo tube etc) was added in the "Trials and Tribble-ations" flashbacks and in TOS-R.
That same K't'inga CG model would again represent a D7 in the Enterprise