ship design

[Shadow]

It's not too far away to see the configuration. Look closely, and you'll notice it's the same like in image 1. It's Devastator.

The armor shows no details in the picture. While the Devastor model may have been modified, it is not canon that ISD mark-Is had there tractor beam sensor removed. The bridge is like that of a mark-I in the picture, however. This Star Destroyer would regared as an attempt to upgrade an ISD mark-I to mark-II, or an entirely different type. In any case it is not an ISD mark-I.

[Cpt_Frank]

The armor shows no details in the picture. While the Devastor model may have been modified, it is not canon that ISD mark-Is had there tractor beam sensor removed. The bridge is like that of a mark-I in the picture, however. This Star Destroyer would regared as an attempt to upgrade an ISD mark-I to mark-II, or an entirely different type. In any case it is not an ISD mark-I.

*sigh*
The Devastator's ventral surface had little detail, compared to the ISD II model, only the terraces and some structuring, by far not as much as the Avenger model.
That's why the dorsal surface looks plain on the pic in comparsion to an ISD II.

The only difference, and you basically admit that, is the absence of the X-shaped array.
Why do you assume it's a completely different subclass, and why don't you just go with the much simpler explanation that the Devastator's X-shaped array was a temporary upgrade?
The Devastator was Vaders ship for hunting Rebels, after all, so it's not unreasonable to assume they added an enhaced tractor beam targetting array, and lateron removed it again since it brought little benefit in regular service (and perhaps even consumed too much energy).