1.2 Normative References reads in part:

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ESA/390 Common I/O-Device and Self-Description, IBM Publication SA22-7204
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I did find:

http://publibz.boulder.ibm.com/cgi-bin/bookmgr/FRAMESET/DZ9AR501/CCONTENTS?DT=19920409154647

but that is 7204-1.

I learned from http://permalink.gmane.org/gmane.comp.emulators.hercules390.general/35757 that there was a version, 7204-2, that is now missing from the IBM site.

Which version of 7204 is this a reference to and is the URL I cite above the correct one?

I suggest adding the URL to the citation. It isn't required but it does make review and adoption of a standard easier if normative references are readily available to everyone.