I can't believe no one even has an opinion about these ducks.
Sorry, RB, I missed this thread/post the first time around.
I am not sure what my opinion is worth, as I have no special expertise in this area, but I would normally presume that something like this is a color changeling rather than some sort of error, because that happens much more frequently and normally does indicate any rarity or added value.
Since the blue-colored examples don't appear to be in pristine condition or anything, and since one was used on a cover, my guess would be that they have been exposed to excess amounts of sunlight, which could have altered the color but left the gum intact. But that is just a guess, as I have no claim of any special expertise in this area.
Regarding your ducks, the images are small, if they were larger we could see the effect upon the smaller colour patches. This would almost certainly reinforce my humble opinion they have been bleached by being displayed near a window. With a population of 1391 +/- a seller in Frazee may well have displayed them and then sold them when stocks ran out.
My 2007 OBP (Flemish version of the "Official Belgian Postage Stamp Catalogue") mentions these imperforate stamps and prices them as complete sets. As far as I can tell, there is no specific mention of the prices being for pairs but I would assume that pairs are the norm, as otherwise it's too difficult to rule out the possibility of a single stamp merely being a normal copy which had the perforation teeth trimmed off.
For the lion rampant issue, the price for the two complete imperforate sets of 16 values was €200 per set (whereas the normal single stamps were priced at €27 and €13 for mint never hinged sets). For the railway parcel post issue, all 6 values combined were valued at €60 as imperforates and €1.50 as mint never hinged normal singles.