[Dennis]

A long time ago, I read that the generic term of SD stood for "Special Dispersion" and in that context, it was a poor man's ED and a very, very poor man's fluorite.

So, the "quality" line appeared to go something like Achromat -> SD -> ED -> Fluorite.

I'm not sure if SD was a marketing ploy to give the consumer a warm feeling that he/she was getting something better than an achromat and closer to ED?

Cheers

Dennis

EDIT:
I might just add that this was from reading UK photography magazines in the early 1990’s. It seems that Televue places SD as you have written, closer to Fluorite.