Honestly Raimey, I don't think Graham gives a hoot about what happened circa 1928. What he knows is what the books say.
It would be interesting to know when such steel stopped being used.
If it's 1939 then you might have a point. But the data is lacking...
I've looked all over for such ZSR, and can't find diddly squat. That would mean that he might be right, it could be a custom spec to a steel maker of some sort. It could have been made by many providers for that matter.
Best regards,
WC-