Fusion recommended this as a better substitute for Ferro Frit 3124 than F-19 (Mar 2021).
It is the same as F-19 it with 2% less CaO.
Ferro Frit 3124 vs Fusion F621/19 at cone 04
The Fusion one is flowing a little more. And it has larger bubbles, so Fusion must be using a different set of raw materials to source the chemistry (materials that either have higher LOI or decompose to produce gases later in the firing). They do not supply the chemistry of Frit F621/19 and it is not shown on their website in 2021. But they recommended it to us as substitute candidate for Ferro Frit 3124.