If I may, I'd like to add what little facts I know, and perhaps am assuming as well to this post, so others can perhaps give thier personal opinions.
It appears Harry+Co is selling a "newer version" HW-19 MK-IV Plinth, which appears to be the old Steel Sub-Chassis, with just one full sheet of .500" Black Acrylic bonded to it (probably via Silicone RTV Caulk) Meaning no seperate Armboards in this instance. Harry probably whipped something up, with some left over stock of the Steel Sub-Chassis plates he had on hand.
The asking price I believe is $300, and as JTgreenlaw has personally confided to me, he states that VPI will cheerfully drill the plinth to whatever Arm one wishes to install.
With that said, and that the drilling seems to be a free accomodation that VPI is willing to do as an act of kindness-consideration for a customer, perhaps $300 is not at all a bad price for this part. I know those steel plates were quite heavy.
I as well suspect these steel plates do have provision for the seperate Armboards, in that there is probably the existing tapped holes on the plate to accomodate such. Mark