On paper, Fusion F-19 has a very similar chemistry to 3124. However, as can be seen here, it is flowing a little more and appears to have a lower surface tension. The glass is also more transparent and the entrained bubbles are bigger. The differences could be partly to Fusion using a different set of raw materials to source the chemistry or differences in their smelting process.
The Fusion one has less surface tension and 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 a 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 a substitute candidate for Ferro Frit 3124.
Ferro Frit 3124
A commonly available borosilicate frit is high in calcium.
Fusion Frit F-19