So, I too have a Pionier 520-Z, but have not sailed it as the only piece missing that I know, was the thwart GrnMtns was kind enough to share his thwart design, so I plan on making one soon.
Although my 520-Z was missing the thwart, it came with a bladder mounted to a piece of wood. Internally it has a couple of semi-circle reinforcements (perpendicular to the length of the device that hold the bladder in place. The bladder an reinforcements are covered in a cotton sleeve, that's a bit ratty. This baffled me as the construction, initially suggested a pad , but I didn't know where it would attach. The item is 1 meter long. Today it dawned on me that this could be used as an outrigger float, or ama. I'll try to post some pix of the device, hopefully the link works!
If it is an ama, then I'll need to find a new bladder fab a new sleeve, and a way to attach the ama to the kayak, as I don't recall seeing any bits for that either. Any assistance identifying this item and what function it serves is appreciated.
PS: tried to post the links as a image and get this error: "It was not possible to determine the dimensions of the image."https://www.flickr.com/photos/146953083@N02/29388872952/in/dateposted-public/https://www.flickr.com/photos/146953083@N02/29463131316/in/dateposted-public/https://www.flickr.com/photos/146953083@N02/29208339350/in/dateposted-public/