When dealing with steel supplies they generally classify the specifications as such when an item has been either discontinued completely or swallowed up in another identifying classification so that the number is not valid anymore. Such as A36 WF beam that is now either A572-50 or A992 (A992 is scheduled to be the final classification when all stocks of A572-50 are depleted). The MTR's will have dual grade on them until such time as the A572 is irrelevant.
If you need such material, normally the former class is not being produced further, but, that doesn't mean it isn't available. But, it may be hard, even very hard, to get. It would be simpler to see what has replaced it or at least comes close to the same specs and redesign your component.
He Is In Control, Have a Great Day, Brent
