(Forgot to add this ear;ier

: I notice that the UK mil CV2666 has a completely extra top mica disc above
: the plates, extending all the way to to the glass, with ceramic inserts, as
: well as the bottom metal disk with ceramic inserts like the RCA 829B.

Like this Mullard version found on the web; it looks identical:
http://www.tnbstore.com.tw/photo/mullard/mu-829b.jpg

"alll the way to the glass" is not quite precise -
little metal fingers actually contact the glass at the edge.