John,
I'd guess with your pen the "8" nib indicates a later replacement nib, typical to Challenger or perhaps Deluxe Challenger from that time. A late production of the jade pen of course is not excluded.
The shot below shows a 1928 flat-top pen in Modernistic Blue celluloid, the pen which would take the name True Blue in 1929 with the appearance of the streamlined version, which would have similar shape to the following three pens.
The black pen looks typical to the 1929-32 "raven black" pen that we tend to lump in with the vaguely defined Thrift pens. Heck, if we didn't have a catalogue page for the True Blue, it would be called a Thrift Pen today.
Next to the black pen are two jade pens. The Jade pen looks just like the black pen save for color. Same imprint.
Note the Jade Pen is not just a relabeled/de-labeld Duofold. This is a long slender pen, a size not made as Duofold.
The last pen is a true Duofold Lady Duofold/Juniorette size. Note the cap-band pattern.
I think your pen, like the black pen and long jade pen in my shot, is another "thrift" pen just in slender short size, just as "raven black and gold" pens and True Blue were made short and long (always though with same size cap, just different barrel length).
My guess is the Jade "Thrift" pens are 1929-1932 or so issue.
Thoughts?
regards
David
Ah, the plot thickens! Yes, I do have a few thoughts. 
I don't like to chalk up nibs that don't fit comfortably into a pattern as a contemporary replacement, as I'm not sure how much nib replacement typically went on back then. With the barrel imprint being such that was used throughout the 1930s, late-1930s manufacture for the jade pen I think is well within the realm of possibility without a stretch of the imagination. However.......
The two things that lead me to a very early 1930s manufacture for the jade pen is the feed and the clip. The feed is a spear feed manufactured without the lucky curve (I've had it apart and can attest it wasn't cut). The clip has the imprint of the patent date. It's this more than the feed that could lead me to early-1930s manufacture.
Let's turn away from the jade to look at your pens for a second. The True Blue is a standard long flat top. The clip doesn't have the patent date, which I would have expected for the late-1928/early 1929 manufacture of the pen. It could just be a trick of the angle of the photo.
Your Raven I would agree looks pretty standard for 1929-1932, with me leaning towards the earlier date due to the patent date on the clip. It's hard to tell from the photo, but I would guess 4-3/4 inches, which would be right for the pen to be a bit shorter than the flat-top True Blue, which would be a bit over 5 inches.
The last pen, agreed, would be a standard Juniorette. You don't mention it, but I'll assume that it's marked "Duofold". Note the clip has no patent date and the "Parker" is a bit higher up on the clip than those with the patent date. I guess that's likely due to the clip being a short one, similar in size to those put on the short thrift pens and vest pocket pens, which are all the same diameter at the top.
Back to the third pen. The length is actually a size made in the Duofold, being the Duofold Special. The Specials seem to be (at least those I've examined) the same diameter as a Junior but of a length that falls in between Junior (4-1/2 to 4-5/8) and a Senior (5-1/8). There's no question that yours is slimmer than a typical Special, looking to be the same as the Lady/Juniorette. I had something similar in a flat top Jade, which had a Lucky Curve imprint (photo at the end). The two cap bands fit in this case, as the Special had the two cap bands like that of the Senior and Junior. But clearly yours has the same imprint as my Jade. I would be curious to know what the nib imprint is and what kind of feed it has. Oh, it's also interesting that it has a patent date on the clip, marking it earlier than later.
So as a further comparison, below is a few streamlined pens shown next to the original Jade in question. Bottom to top, they are:
1. The Jade from the original post. Length 4-1/2 inches.
2. Standard streamlined Duofold Junior. Length 4-5/8 inches.
3. Streamlined Duofold Special (so marked). Length 4-15/16 inches. (I just noticed something in the imprint that I'll reserve for another active thread).
4. Raven. Length 4-15/16 inches.
I bring this photo forth just so we could see all the different pens lined up right next to each other to judge length and thickness.
So where does that leave us? I think we agree that this is a depression era "thrift" / "$3.50" pen made in the Jade plastic typically reserved only for Duofolds, which would make it off-catalog in my opinion. Your third pen certainly confirms that there was a long and short variety made, just as was done with some of the thrift models. Where I'm not ready to agree is the possible date of manufacture. There are features that tell me early-1930s and there are features that tell me late-1930s. Still, pretty cool to have these come to light.
Oh, BTW, here's the photo of the long thin flat top Jade Lucky Curve. Sorry I don't have more detail, but I don't own the pen any longer.
