I think I may still have a partial can of that stuff on the shelf somewhere.
I used it back in the 60's or early 70's.
Best I remember it was like most of the fillers. Heavy bodied 'mud' suspended in some combination of oil & maybe some thinners. IIRC it smelled alot like Linseed.
The color I remember is a rust brown

You spend more time mixing it to get the mud which are likely the Earth Colors used in oil paints into an even suspension of liquid.
You spread it onto the wood . Let it dry for a time. Work it across grain to push it into the pores. Let it dry which takes a long time.
Then sand it off to the surface of the wood.
If the pores are not completely filled,,do it again.

The orig was gritty feeling when you rubbed it gently betw your fingers. I suspect that was from the Natural materials used for the colors.
Something has to fill those pores!
But that played Hell with checkering tools I do remember that.

I switched to sanding in the finish to fill pores a long time ago.

I've never used the 'New' Herter's formula Stock Filler