Just checked a couple of my slrs w/o film in them
Nikon Fm and FE---smooth ecept for 2 recessed screw heads
Pentax Program Plus and Ricoh XRP---dimpled
I have a Pentax ZX5 and a PZ1P - I'm fairly sure they both have dimpled platens, too. Both are loaded at the moment or I'd check, too. I've used HIE in both, though and haven't seen the kind of pattern you describe. I have, however, seen something different with the PZ1p - a lengthened triangle. It's only shown up with HIE (in a light sky) and on slide film that was over-X-rayed. When it's empty, I'll check it out.
Some more info to add to the list so far...
MZ-S (probably not good for IR anyway because of the data imprint)
Spotmatic SP500 (and presumably SP1000)
Konica Auto S2
Nikon F3 (has 2 screw heads like bohica mentioned, but these are outside of the image area)