Thanks for replying, tooki - yes I agree, the DS21 must be older. I was sure I'd seen the DS22 (which is pretty much the same) on a compatibility chart for the WMD-3, but now I can't seem to that either. And you're right - both the DS80 and the DSX80 are compatible, I'll save that file for future reference - thanks!
I won't try to Frankenstein a DS80 heater into the DS21 handle unless I find evidence that they're compatible.