Hiya Rod, thanks for your considerable input on this.
A couple things:
Mine is indeed perf 10.5
The clouds are, I believe, actually different in the later version of the stamp, and not quite like the PT79 design. I am including a screen cap from Stampworld (not that I trust Stampworld any more than Scott), showing all the various denominations from that later set. All of the ones with this particular design have the same cloud pattern and the inscription in the margin, although you cannot see the 25k because of such a small margin.
I do believe it is the RA208, design, perf and color at least. But, I would also think that such an "error" would have been recorded somewhere, so the conclusion that it is something privately produced, or printers waste is much more logical, and it is how I will label it in my album, until other information might come to light.