I picked up two stock runs, a white 172g and a swirly purple 174g. I ordered them from discsport in Sweden though, haven't seen anywhere in the US have them in stock for long enough to have a chance at getting one.
Both have a board flat top with little to no shoulder. They have a similar bottom rim shape to a zone, but with a higher PLH, but feel at least as good in the hand as a Zone to me. I've only played one full round and done some field work with them, but I'd be kinda surprised if the Järn doesn't kick both zones, the A2, and the Z Buzzz OS all out of my bag. It's just fast enough to cover the Buzzz OS lines with better hand feel, and OS enough to do everything the Zone or A2 could.
I threw harps for a long time before switching to the zone, have thrown a number of other OS approach discs, and have definitely developed some favorites for that slot. There's a lot of great discs for it, but for my game, the Järn is the best one I've tried. The little bit of extra speed makes it so versatile! I can smash it full power and get a straight to heavy fade flight still, but push that out to a bit over 300'. Or, I can throw very touchy upshots, or short dramatic flexes, or controlled skip shots around a corner, or those stiff headwind putts, or overtorque a FH without much consequence, or blah, blah, blah. I could talk for way too long about how much I like the Järn.
In reference to the question about the different font size on the rim and the 6 ejector pin marks on the bottom of the flight plate, I highly doubt Lat64 is molding these. I ordered an X-out Lots to try along with the Järns, and it too has the larger font size (though still on the rim for the Lots), the 6 ejector pin marks, and the slightly different center sprue (where the plastic is injected into the mold). I would guess the hypothesis that Kastaplast got a new molding machine is likely to be correct. The difference in if there are those 6 little circles, or at least how prominent they are, is probably due entirely to how long each disc was allowed to cool in the mold after injection, before those pins push the disc out of the mold along with the stiffness of the plastic used.
All right, sorry for the wall of text, I just got excited about these!