I don't know the official answer, but a way to guesstimate a quasi-sane 
answer:

Pick whatever machine was made available 3 years after OS/390 2.5 became 
available.
Any machine available after that would most likely not have had any 
exploitation support provided as far back as a release 3 years old, but 
might have had toleration support.
Add another 3 years and come up with that machine and you'll likely not 
have had toleration support either as far back as OS/390 2.5.

That would then be the machine that you should not be at all surprised if 
OS/390 2.5 did not run on.

Although I suppose it is possible (I do not recall) that back then we 
provided support "further back" when a new machine came along.

Peter Relson
z/OS Core Technology Design


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