Gray cast iron threaded fittings in small sizes (1/2-2") are no longer available, or if they are, they're rare enough that I've never seen one in the ~ 25 yrs I've been dealing with fittings on a daily basis.
If you find a 150# threaded fitting which sticks to a magnet, it is almost certain to be malleable iron, whether it is galvanized or black (not galvanized). These fittings are also made in cast stainless steels such as CF8 (304) and CF8M (316).
3000# threaded fittings are generally A105 carbon steel forgings, not castings.
300# malleable iron fittings do exist, and just to make confusion more likely, they have dimensions very similar to 3000# forged steel fittings.
That link that John R Baker posted is very confusing. I've never heard of "black iron pipe" before, and suspect that it is a misuse of the "black iron" description of non-galvanized black malleable iron fittings, misapplied to the pipe which is STEEL I am unaware of an ASTM designation for anything called "black iron pipe". I do know what centrifugally cast iron pipe is, and it is still used in drainage applications, but it is NOT joined by threading.
Steel pipe here for pressure applications is typically A53 welded (or sometimes seamless) or A106 seamless. Those are steel, not "iron". There are steel pipe and round hollow structural steel (HSS) grades available for use in non-piping (structural) applications which are identical in dimensions to pressure pipe, but which use much less reliable "furnace welding" or other means to form the long seam, so you do have to be careful making assumptions about spec/grade based on dimensions alone.
