I handled the pen in DC, I believe it to be the real deal, it is definitely a Parker produced piece, I would only figure that they had an unfinished JKS barrel laying around in a parts bin, and decided to fit the experimental filler to it.
Or even another possibility, maybe this thing was made earlier than the patent, and Parker just decided late of 1929, to patent it to protect it from another pen company using the design, say... Chilton? it is exactly the same as a chilton filler from the long island days, and the parker patent predates the chilton patent by a year or two. I just wonder why Parker didn't go after Chilton to protect the patent, and to another extent, Sheaffer?
Actually, the mechanism of the patent is different than that of the LIC Chiltons. The key feature is a porous packing unit at the rear of the barrel. So, air compresses under pressure and is slowly released through the porous packing gasket. The Chilton packing gasket is airtight, more or less, and air is released through the finger hole in the end cap (most variants). The self-release of air most closely resembles the Chilton variant where the end cap is loose and is pressed to the metal barrel on the downstroke, or the Seth Crocker filler where the same thing is accomplished by an internal gasket (at least when either of those mechanisms decides to work). Note that although there is no hole in the end cap in the patent, there is a hole present in the end cap of the actual pen. This suggests that, like some of the Chiltons/Seth Crockers with the mechanisms mentioned, the JKS was converted to the LIC Chilton style mechanism, which would allow use of a non-porous gasket, since the finger release would allow air pressure to escape.
If the hole in the end cap of the pen is an integral part of the filler, it is not the filler in the patent.