Interesting. I wonder if there are any examples of a Parcel Post Postage Due Stamp used for regular postage. It would be an interesting conversation piece to say the least!
Found another piece of information on this. Advance to page 19 of this presentation, wherein it states that Parcel Post Postage Dues were (after the exclusive use period) used as regular postage due stamps but that some "illegally used" them as regular postage:
http://postalmuseum.si.edu/symposiu...entation.pdfIt seems someone is wrong here. Either the presenter at the Postal Museum Symposium in 2011 or the listing in the Scott and Mystic catalog references recited earlier.
I suspect that the correct reference is that Parcel Post Postage Due stamps were not valid for regular mail, but only for regular postage due (at least that would make sense to me) but the post office has been known to do stranger things.
But if that's the true case, then both the Scott and Mystic catalogs are wrong in their comments!