Well, it had to happen eventually. The 8664A sig gen I bought arrived with severe damage. The unit had to have been dropped from some height, because the right rack handle is destroyed and the RF output is bent severely.
The obvious damage. The other rack handle is bent too. The instrument seems functional, but it had a message "hardware failure 8" which is now gone, that I am trying to find a definition of.
I have an 8644B, which appears to use similar construction. I just saw your post on the HPAK mailing list, but figure I'll answer here too as it's easier to go back and forth IMO. One of the options you mention is for low emission; those instruments get a one-piece sleeve case, rather than the more common top and bottom, easily removable panels. If you peel the vinyl trim from the collar just behind the front panel, you'll discover a bunch of torx screws. Remove them, and you should then be able to get the cabinet off. (You'll need to remove back bumpers from the instrument, too, if they're present.)
Here are some photos of mine; unfortunately I didn't have reason to take many of the case but you can see the sleeve enclosure retaining screws and the clamping bars.
Screws across top of instrument:
Full complement of screws and clamping bars:
Instrument with cover removed:
Good luck with it, it's a fine piece of hardware!
-Pat