Allow me to throw my $.02 in here. Although I am certainly no defender of Chinese 'quality' I would never use cast iron fittings in an air system. I work for a plumbing & heating distribution company that sells all types of fittings. The 'old timers' who do boiler work like to use cast fittings. Why? Because you can smash them with a hammer if you have to do some piping changes. Malleable iron fittings could realistically never be smashed and they have no 'casting hole' issues that the OP discovered. Malleable fitttings are rated:
Standard Class 150 Specifications:
ANSI B1.20.1, Threads, B 16.3, Dimensions, Pressure Rating
ASTM A197, Material. A153, Galvanizing
Federal Spec: WWP 521
MSDS Malleable
NSF Listing
Pressure Ratings:150 psig - Saturated Steam
300 psig - At 150 Degrees W. O. G.
Where W.O.G. stands for water, oil or gas service
Cast fittings have significantly lower ratings:
Standard Class 125 Specifications:
ANSI B1.20.1, Threads, B 16.4, Dimensions, Pressure Rating
ASTM A126, Material. A153, Galvanizing
Federal Spec: WWP 521
MSDS Gray Cast
NSF Listing
Pressure Ratings:125 psig - Saturated Steam
175 psig - At 150 Degrees W. O. G.
Again, not defending Chinese quality, 'sand holes' from the casting process is not that all unusual an occurance in cast fittings - no matter where they are made.