The back looks like others I have found online but haven't found a photo of what the inside looks like. I'm assuming there's supposed to be 2 little tabs molded into the front panel that have been snapped off by someone being overzealous.
Super, now we know that this is just a common design issue with this and newer Keithley DMMs - yep these two tabs are broken and that's why fuse holder was missing.
'Just' need to restore them somehow. My option would be to cut place where they were supposed to be and put new ones (plate made maybe from FR4, or maybe smth metal). Plate - better to say circular ring with these tabs inside so you just put it on the back, maybe glue a bit, but it then really holds because of the spring tension.
Don't remember seeing good solution on forum, but maybe it's already there.