Rich Yoder asked:
"Were Head blocks outlawed ?"
Bill Kelly added:
"With my collection of AAR Interchange Rules the best I can say is possibly as early as 1 Jan 38 but definitely by 1 Jan 43 tank cars having head block anchorage were prohibited in interchange."
There was an extension to at least January 1, 1939. The Mechanical Division of the AAR did not hold their annual meeting in 1938, and the Arbitration Committee's special report (1938) within the 1939 Proceedings contains no mention of an extension, but there may have been one (via supplement) as such events were not always included within the annual reports.
As of January 1, 1941, tank cars which were equipped with head blocks were absolutely prohibited in interchange. Therefore, the final date is either January 1, 1939 or there may be and extension (up to) January 1, 1941.
I am confident that Gene Green will nail down the exact date when he has a minute to check his collection.