George, you are correct-- pistons are virtually non-existent (I remember seeing a somewhat surface-rusty "NOS" set on fleabay a few years ago with a buy-it-now of $450 for the set of six pistons-- got a nervous laugh out of me)
MBP, I bought the kit from Ray Paul some time back (don't quite remember how much, but somewhere in the $30-$50 range), and it came in a small zip-lock bag with a label printed with the following:
AS SP2511
BBGIF
MADE IN ENGLAND
I believe "SP2511" is the actual Lucas-Girling part number, and these pistons were also used on some other cars of the era (Jaguar, etc).
The kit came with six "O" rings (one for each piston: two large and four small) and dust-covers (two large, and two combo "8" shaped covers for the 4 smaller pistons).
Hope this helps,
Waqas in Austin, Texas