Dan,
Thanks for the suggestion.
I used to have one, must be in a box somewhere in the garage, but I found the single bolt seatpin it fits to have the same problem of discrete jumps in angle which is ok if one of the angles suits, but otherwise not so ok. Maybe I had a cheap seatpin, I don't remember now.
I am spoiled on my own bike by a lovely lugged Nitto S84 double bolt job. Infinite angle adjustment for me, sadly they are too short to put one on my wife's bike
The Brompton pentaclip is a work of art though. It fits single rail saddles, old plain top seatpins and gives infinite angle adjustment with just an allen key. You can set the pentaclip backwards or forwards as suits you with the saddle position, there is also an upper and a lower position for the saddle rails.
I will be selling this old B66s saddle off cheap, so no real incentive for me to buy extra kit.
If it doesn't sell then it just goes in a box in the garage - I must have the making of another bike in there in old bits.
I run a cycle club at my local church and any unsold bits will get reused there eventually.
Julian.