Yes, manganese phosphate, I think, from Allegheny Arsenal.

At first I tried phosphating on a propane camp stove, but found it impossible to control the temperature at 200 degrees, just below boiling. When the phosphating solution boiled, the chemicals precipitated out of solution, which didn't totally ruin it, but wasn't what's wanted.

Found a large electric fry skillet at a garage sale, and this works very well with excellent temperature control and a double stainless pot set up, a pot within a pot, as in photo.

Still used the camp stove for boiling water as a pre-heat before parts dipping in the phosphating solution.

Oiling after phosphating is recommended, but I don't think it helps much. Besides, on body bolts to be painted, you wouldn't want oil on those, which leads me to guess that original phosphated fasteners probably weren't oiled.