@melm: When I used the term "solid Delrin" platter, it was in reference to not having a 2-layer construction (aluminum and Delrin, for instance). I’ve never seen the thick (2"?) black platter in person, and have suspected (but did not know for a fact) that THAT platter may have had the lead layer bonded onto its’ bottom.
The HW-19 Mk.4, Aries Black Knight, and and original TNT had that thick black platter, made of---sorry to insist---Delrin. I know Harry and lots of others used the term Acrylic interchangeably with Delrin, but I assure you the black platter material is in fact Delrin, not Acrylic.
At a big commercial plastics warehouse/machining facility in Portland Oregon I took my Aries 1 arm mounting plate into to have duplicated, and took along my aluminum/lead/Delrin platter. They confirmed that the black material was indeed Delrin. They brought out both black Acrylic and Delrin for me to look at, and the Delrin looked and sounded just like the Delrin on my Aries platter. Even raw Delrin has a completely different mass consistency and resonant sonic characteristic than Acrylic, which is inherently glossy and lower in mass. Delrin looks almost like stone, Acrylic obviously man-made.
All the long-time posters on the VPI Forum refer to the black VPI platter as Delrin, which is where I learned of the term. I really like the Delrin-topped VPI platters for their PVC/Delrin interface characteristics, and wouldn’t have a VPI without one. But there are lots of happy VPI owners with the current solid aluminum platters.