Hi Brusson, Hmm, What I personally found myself, was more, or less the exact opposite.
I believe my second hand Mk-IV upgraded Platter to be an older version as well, weighing in at axactly 18lbs.
While the Mk-IV Platter I had gotten had a much more sturdy, larger diameter Bearing, and Bearing Well, versus the original Jr Platter, I had noticed the Spindle itslef to be just a tad shorter than the Jr.
Both were 1/4"-20 threaded for Clamps. The Jr Bearing Well I had appeared to have a Brass Bushing at the top near the Bearing Well Flange, While my MK-IV Platter Bearing Well, has a Bushing made out of some sort of Black material, and I'm assuming it is Delrin, and not some type of Metal.
I'm almost positive that all Jr Platters had the bearing-spindle pressed into the Platter, while the Mk-IV Platter, such as I have, the Platter just sits on top of the Bearing-Bearing Flange, and it's massive weight keep eveything coupled up.
I notice on my MK-IV Platter itself, 3 Drilled Holes in its center, with three Metal Pins-Screws installed (The supposed 3-point support system I guess), which appear to be threaded into-through the Platter, and with Allen Head screw heads on the top-side of the Platter. Mine appear to be perfectly flush with the underside of the platter so I could not gain any spindle length by screwing in-adjusting the pins. No doubt probably not a good idea to monkey with anyway, as one would probably wind up with Platter Wobble.
I was under the premonition that later versions of the MK-IV Platters were 20lbs, and then also moved on to an Inverted Bearing? Which is yours? Mark