It's not certain that it's got the normal 9915 boards inside, even though the front and rear panels look identical. Actually, on a 9915 without the Operator Interface boardm, do you get a back panel with the holes present but blanked off, or do you get a back panel without holes? As the back panel is part odf the chassis, I would have guessed the former (making it possible to field-upgrade to the Operator Interface later), but I am not sure. If it is a 9915 in there, it's possible the system ROMs are non-standard. Or that a special ROM is fitted to one of the ROM module connectors. Well, the normal way to link one to a 9915 is to put an HPIB interface in one of the slots and hang a 1350 or 1351 'graphics translator' off it. I wonder if this 5181 is designed to have an HPIB interface in one of the slots. It may be designed to act as a listener/talker only (not a controller), and to connect to some other computer as a storage device. The hardware, assuming it is 9915-based is capable of that, obviously. -tony