EXTENDED DESCRIPTIONS
U429 IN A "RED-ORANGE" SHADE
If you've seen as many envelopes as I have, then you know that for certain long-running definitive issues - and it doesn't get any more long-running than U429 - there exists a wide variety of shades. Very few are major enough to deserve a listing. This should be one of them, and there is a precedent, as I'll explain. 
The "normal" U429 is a solid, reddish "carmine". That's the image at the left.
The shade in question is the 2nd image. I'm calling it red-orange. I've seen it before, but only on cut squares, and I've never owned a complete envelope. It's a U429, Size 23, Knife 112, Watermark 20, UPSS # 2238/20.
The 3rd image is a Cut square of a U411g. The envelope whence it came would have been a UPSS# 1761a or a 1762a, Watermark 19 or 20. UPSS calls it orange-red. Whatever....
The 4th image is a U324, which UPSS calls either Scarlet or Orange, and I can't tell you which this one is.
Now, putting aside the 4th image, which I threw in there just to be ornery, and putting aside the issue of whether you want to call it orange-red or red-orange...the 3rd image is on a watermark 19 or 20 paper, and this envelope I'm offering here is alsoon a watermark 20 paper.
I believe the color is NOT an error; it's a variety, and it's some kind of "hold-over" from the printing of the U411g. It deserves a listing.