Note the temperature.

Many modern fixers will work rapidly at 48C. In fact, you better have a very hard film.

The Antimony salt probably helps harden here just as Zirconium salts can under some conditions.

I could probably formulat a fixer that did this at 75 degrees as well, but the price would be high. (well, this one does not come cheap either I guess.)

Interesting formula Ian.

PE