...Which is basically what the TNG TM Type 9a is, only with different nacelles.
That is, the cockpit in "Skin of Evil" is of course common with Type 7 - but the exterior of Type 9 would nicely cater for that, since the exact window shape never is seen much from the inside in any use of that set/prop (least of all in "Skin of Evil"), and common cockpits across a whole fleet of designs would make sense. In turn, the "Skin of Evil" stern jutting out of the rock doesn't bear the slightest resemblance to the stern of Type 7, but it doesn't take all that much squinting to see said butt contours in the ambiguous art for the TM craft.
It's a chore to load cargo onto a Type 7, which has no "real" doors on the prop but ostensibly has a narrow ramp in the middle of the bow (too narrow for cargo), then two gullwingish side doors, at least in most models (too high-mounted for cargo), and potentially a stern door with the impulse engine mounted in the middle of it (probably not the best way to bring in bulky items even if the door is wider than the bow ramp). A "companion" cargo design as depicted in the TM sure makes sense, then.
Although whether the fancy ovoidish depression at the stern of the "Skin of Evil" craft is a practical cargo door is again debatable... It might work better as a big rear window,
Danube class style. (Perhaps there are optional stern modules, some with big rear ramps, others with this nice window?

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Timo Saloniemi