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.