Mosvalve MV-962...different versions?

[scifishredder]

I noticed some different versions of the MV-962 and would greatly appreciate if anyone familiar with these power amps could provide some info.
In the following pics, one of them has a bigger enclosure in which the sides extend out close to the edges of the rack ears. The enclosure on the other unit looks a lot smaller with longer rack ears. Is one of them a later model than the other? Was the change in enclosure size a design improvement of some kind? If so, what improvements were made between the two?
If I were to purchase one of these used, is there one which would be a better choice and why?

Thanks