The compound, or filler could be something like "Smooth-on", a compound used for repairs and sealing of cast iron. It was mainly used for things like sealing freeze cracks in engine water jackets or cracks in cast iron low pressure applications. Or, the filler could have been a shop-mixed thing, and any number of old texts have instructions on how to mix these sorts of fillers. Stuff like 'litharge', and even Portland Cement mixed with paint were used.
As to removing the filler in a timely and easy manner, I'd suggest using an air needle scaler. You could remove the filler with a welder's hand chipping hammer, using the pointed end and just pecking away at the filler deposits. The air needle scaler is quicker and does a better and more thorough job. An air needle scaler needs a shop air compressor to supply compressed air to run it. It will leave the cast iron in a clean 'frosted' condition. Blow or vacuum off the remaining dust and debris and the castings are ready for primer. Things like an angle grinder with a cup wire brush will load up with the filler or throw a lot of particulates into the air. The filler may be 'friable' (able to be crumbled), so the needle scaler will make short work of it.
Another thought is to leave the filler deposits in place. Use automotive body sanders to smooth out these filler deposits if necessary. Build up on them with modern auto body fillers, and finish by sanding. Removing the old filler may disclose really rough castings with porosities or 'blowholes' from the foundry, needing to be refilled to slick them off for the new paint. Back when the Becker mill was made, there were no handy air tools or electric tools like angle grinders, needle scalers and air sanders. The castings were 'snagged' at the foundry on large coarse grinding wheels, and sprues and gates and mold flash were chipped off with hammers and chisels and given a lick and promise with the coarsest and largest files imaginable. After that, it was a case of filling the holes and rough areas on the castings with whatever the flavor of the week was for filler. As I wrote, this filler may have been shop-mixed by the painters, and could have any likely substances that were at hand mixed in.
I've seen some fillers on old machinery castings that broke up readily when hit with an air needle scaler, nothing oily about them. Great bonding to the 'substrate' (fancy term for what a sealant, paint, or filler is applied to). However, these old fillers were brittle when hit with the needle scaler or chipping hammer and popped off in chunks.
I second Joe's suggestion to try a needle scaler, or needlegun as I've always called it. But with a caveat. I almost never use the needlegun alone. I use it along with an angle grinder with wire wheel. The two together work much faster and better than either tool alone.
I also agree with Joe's other idea of leaving it in place. It won't hurt anything under your paint.